The RPI is a much better server, even with a 4 you gotta struggle with it to get streaming video. If you have a lot of firefox tabs open right now, look elsewhere.

permacomputing tends to be much more about the software than the hardware.

Always get the four freedoms first https://www.gnu.org/philosophy/free-sw.en.html

I used a Pi 2 for a time as a daily driver when my laptop had caught fire and it was able to handle stuff without much difficulty, though internet browsing would significantly take it down, making it freeze and require I take a break before returning. I ended up using it with my old tablet and was able to get through an entire semester of college, mind you this was back in 2015. I would easily imagine that a Pi 4 could do enough for you, especially if you used it with a remote connection distro specifically, setting up a server that you would pull your desktop from for your classwork.

I also have several old PCs that I’ve restored and have used and found them able to do everything I need and sometimes can even do more with less. The old Dell Optiplex SFFs are still being used everywhere and being thrown out, and are easy to get parts for or upgrade up to make it a good daily driver or even a server, I got three of them that were being thrown out by a hospital a few years ago, the hospital throwing them out as a “it has been 3 years so we gotta upgrade” policy. I ended up getting parts and we have them in use at home.