With respect, if you cannot notice a difference between 720p and 4k on a 4k compatible screen, then you do indeed have something wrong with your eyes.

Either your eyes are bad or the files you've watched. There's an absolutely enormous difference between 720p and proper 4k with a decent bitrate. Also a decent display with good HDR can add a lot as well.

Resolution isn't actually important. Dots per inch is, but also it needs to be scaled by distance. Low DPI isn't noticeable if you're far away, but even high DPI is when you're close to it. It might be less that your eyes are going bad and more that you're sitting further away from your screen or your screen is really small.

I have an old PC with a gtx 1070 with 2tb ssd and 24 GB of hard drives, all bought before the AI price hikes of course. I am already at a net win.

If I pay for YouTube premium, Netflix, disney, amazon prime, HBO max, spotify. I have to spend around 70 euros a month. (4k content or family pack)

Here's a repo to get you started, though you'll need to either do some workaround in SteamOS to get it to stick since it is immutable (Steam Deck/Machine community has a couple ways of doing it), or use a regular game optimized distro like CachyOS: https://github.com/WayDroid-ATV/waydroid-androidtv-builds

Make Android TV shortcuts for apps that don't normally work on Android, but have gamepad support: https://atvlauncher.trekgonewild.de/

You could also just do regular Waydroid, but some apps won't give you the TV optimized version with gamepad/remote support, then you need to use something like this or Mantis: https://github.com/keymapperorg/KeyMapper