I plan on using proton until I hate myself enough to run my own email server

By definition an email server is not under your control, so the question of whether or not it runs FOSS is a bit moot and in any case impossible to verify.

In terms of privacy-respecting email hosting, Proton, Posteo, and Mailbox all spring to mind.

"Graphic design is my passion"

Check disroot
https://www.makeuseof.com/tag/disroot-encrypted-email-cloud/

It’s not completely FOSS, but I run Port87, which is quite a bit FOSS. It uses Haraka as its SMTP server, SvelteKit as its server framework, Nymph.js as its database layer, Svelte as its frontend framework, and Svelte Material UI as its UI framework.

The ones that I created and maintain are:

• Nymph.js
• SMUI

The base app layout is also available on GitHub.