Youtube — try Newpipe for background playback. There is a version with Sponsorblock, I believe

Gallery — I use the Fossify one. You'll find a lot of other FOSS utility alternatives searching for Fossify.

Media players — I waffle between different ones, but I always have VLC installed as a fallback. It feels clunky on Android, but it will play almost anything.

Don't forget migrating your accounts in various online services if you used Gmail or Google login for registration.

For mail address, it is best practice to register your own domain and use any reputable mail service provider that allows to use custom domains. This way your address is not tied to the provider and you still own it in case of any issues with the service provider. This means you don't loose access to the online services when Gmail (or anyone else) decides to block you.

See my Awesome Android Apps list.

Here's what I use:

• YouTube:
  • LibreTube has really nice UI/UX and integrates SponsorBlock and DeArrow. It relies on Piped for fetching subscriptions (since the newest version it will play just fine with a broken Piped server though).
  • NewPipeX is what I use when LibreTube doesn't work and for downloading YouTube content. It is a fork of NewPipe which integrates SponsorBlock.
• Mail:
  • I use posteo.de as a provider, but it isn't fully liked here due to it not supporting custom domains.
  • mailbox.org is another solid provider (with support for custom domains AFAIK) if you don't want to use Proton for E-Mail.
  • FairEmail is my mobile client of choice (Geary or Thunderbird are good for Desktop).
  • k9 Mail is an equally good choice.
• Cloud Storage:
  • For Cloud Storage I prefer the selfhosted Seafile.
  • If you just want to share files temporarily, Wormhole is a good tool for that.
• Gallery App:
  • Aves Libre is solid if you don't want to selfhost, but it doesn't have fancy features like automatically (od manually) detecting faces.
  • Immich is pretty much the selfhosted Gallery app.
• Video Player:
  • Just (Video) Player is what I use, but honestly the stock player should be fine.
  • VLC will play pretty much anything you throw at it, but I don't like its UI that much.
• Music Player:
  • Vanilla Music is alright, but nothing special. Musicolet, while not FOSS, is a privacy respecting Music Player that has a bunch of handy features, most notable one for me is the ability to have multiple queues.
• 2FA App:
  • Aegis has never let me down, although you should keep in mind it doesn't have any sort of cloud sync.

For password manager and weather I use the same apps as you.

I run a Pixel with GrapheneOS. I actually also came from Samsung, and for me there are quite a few creature comforts missing that I didn't even think about (eg. Samsung Dex, DP altmode, I really like the One UI Dialer, Miracast, Brightness Slider in notifications, switching recents and back button, headphone jack, SD card slot, ...).

As for the Pixel being worth it, I'm ok with it as I was due for an upgrade anyway (Galaxy S10 -> Pixel 6). I'd recommend, if you value your privacy and are fine with losing some Samsung features, to either go all the way and upgrade to a new Pixel when your S23 gets old / dies or buy a cheap used Pixel (Pixel 6 and 7 currently has pretty decent value) to just test it out.

I use a Fairphone with e/os, which works fairly well as an os. Regular updates and they listen to user feedback. And e/os has an app lounge where you can download apps from the play store (without a google account) and f-droid, which is very easy. And the fairphone is great with interchangeable parts.

For maps I use a combo of organic maps and Magic earth. Magic Earth isn't Foss but is privacy focused and has traffic alerts for driving.

YouTube - Newpipe

And for mail, passwordmanager, VPN, calendar, and drive I use Proton.

Here are the ones I use:

• YouTube:

This is the hardest and you might need to hop a lot. But, these are the ones I have: FreeTube, Grayjay, LibreTube, NewPipe, PipePipe, and if none of these work, then YTDLnis (yt-dlp client)

• Mail:

ProtonMail (Tuta is heavily suggested too, but I personally have never used it)

• Cloud Storage:

Proton Drive (although I don't use cloud storage much)

• Gallery:

Stock and AvesLibre (I heard Immich is good too, but I cannot afford self hosting atm)

• Video Player

VLC

• Audio Player

VLC

• 2FA

Proton Pass and KeepassXC

• Mail App

ProtonMail and K9Mail

• Password Manager

Proton Pass and KeepassXC

• Weather app

Breezy Weather

Cloud storage: kDrive

If you're considering self hosting, Immich is an amazingly complete replacement for Google Photos.

Firefox has a pretty good password manager built in.

I use k9 mail, which I really like, although a lot of people like FairEmail (both FOSS)

Also check out YouApps in addition to fossify for a different suite of useful, well designed apps

Ente is an awesome Google Photos alternative.

RiMusic is a great Youtube music frontend.

Tubular is a NewPipe fork with sponsorblock that I currently enjoy. Used LibreTube but Piped was fucked for me for awhile.

For email migration / Proton:

• Proton has an import tool but I think it's still for paid customers only. You could pay for a 1 month subscription and do your thing.
• The import tool has labels matching, pay attention to them if you want to migrate them too. If you have multiple addresses, create a label for each when importing so you'll have an easy time identifying/filtering later.
• I would encourage a paid Proton subscription if you can afford it, for its extra features. I.e. unlimited folders / labels; I also use the Export tool from time to time to backup all Proton email messages offline.
• When changing your email address for accounts, you need to make sure all are accounted for in your password manager, and use a status for each of them (ie To migrate, Migrated, Cannot migrate / To delete, Cannot migrate / create new). This is going to be a tedious process, but it will be rewarding at the end.
• Don't leave your accounts that you cannot migrate in the air (ie they don't have a change email option), even if they're not important. Delete them. You might have to contact support on some of them to try to change email or request deletion. Consider this a spring cleaning and make the efort.
• When deleting accounts, use the GDPR option if possible / you're in the EU.
• Keep your old Gmail around for some time to catch any accounts linked to it that you might have missed. As somebody else mentioned, there might be some for which you used Gmail login and those are easy to miss, especially if they don't send any emails. You won't be able to recover them without access to your Google account.
• I wouldn't bother with forwarding emails (why let Google know of your new identity?). Delete emails already imported. Use the import tool multiple times to import any new ones.
• When you do the email address migration, even if you didn't have multiple email addresses in Gmail, this could be a good time to separate online identities and have multiple addresses and/or aliases in Proton (ie 1. for personal/official/utilities accounts - your real identity, 2. shopping - still real identity, but these might be spammier, 3. rest/disposable/not tied to your real info/no payment enrolled; even more, depending on your use case). Any Proton paid plan allows you to have multiple addresses under the same login (10 for Mail Plus for example).
• Personal opinion: Proton is awesome. Every year, even on the cheapest Mail Plus plan, Proton awards a 1GB Storage Bonus to all paying users.
• The free plan has a limit of only 1 custom filter (they used to limit them for Mail Plus too, some time ago). To bypass that (Proton even encourages it because it's more efficient for their servers), learn Sieve Filters, and that way you can group multiple filters into one sieve (or have all of them in one sieve, if on the free plan). You can use comments in sieve filters.
• Proton supports the "+" in the address, just like Gmail does. It's a quick way of creating aliases.
• The Proton password manager also has some feature of creating aliases (for paid plans) - they call them "hide-my-email aliases"; but it's limited for the lower tier plans (10 for Mail Plus), and maybe you wouldn't want to bother with it since you won't be using it as your actual password manager.
• Something I learned the hard way, don't use a short 3-4 characters username / email address (probably hard to find any available anymore, as Proton exists for some time); it will attract more spam from spammers randomly generating email addresses / generating them from a dictionary.

For Youtube, on Android:

• I use Youtube in Brave browser, which for the time being can still block the ads, and also keeps playing in the background.
• When I want to avoid Youtube, for Youtube links, I use UntrackMe (F-Droid), which (among others) redirects to an Invidious instance opened in the browser. Initially I installed it for Twitter, but Nitter doesn't work anymore.

Cloud storage:

• I'd go the self hosted route - NextCloud + DAVx5 (contacts sync), and VPN to access it when out of home (if needed; otherwise, set it to sync over unmetered WiFi connections only, and mark your home WiFi as unmetered). But this is me - I could probably safely use Proton Drive, but wouldn't have the same flexibility and would force me to a higher cost plan. For you, this means entering homelab territory and it gets complicated. But IF you do, there are other self-hosting apps you could benefit from (ie PiHole, Jellyfin, Home Assistant if you're into home automation, etc).

2FA app:

• Never tried Authy, but I use Aegis and it's good. Open source, it has backup, export, custom icons for entries, (bulk) import via QR, etc.

Video player:

• You could try VLC if you need subtitles support.

As Video player VLC, it's fantastic.

Samsung pay will be the most difficult one to replace. Basically: there is no FOSS alternative, because any tap-to-pay app has to pay for the licenses for using the cards, and apparently (I wish I knew better) theses licenses also go by country/area. And because it's banking I can imagine they are not cheap. So, only a big company could you offer that. Anyway, there are some alternatives:

• Many people that switched to GrapheneOS use Curve, a British payment company. They are much better than Google or Samsung in terms of privacy (once you have account you can write them an e-mail for requesting your data not to be used for marketing. At least in my case they answered and confirmed). But they only operate in Europe. And for creating an account you have to go through ID verification. You read that in many cases people are rejected.
• A new thing in its initial steps is Wero. It will offer tap-to-pay, but it is still being rolled out and will operate only in Europe. At present it works only partially in a few countries.
• Garmin users use Quickopay. But this one I even haven't tried it, as the reviews about reliability are terrible. Maybe with time they improve the app

I only know European option, as I live in Europe. I don't know if this helps you.