It's open source and gives you a lot of fantastic features that are locked behind paywalls and more on plex. But I will be honest it's not nearly as simple and setting it up so I could use it outside of my home network took a week of figuring out issues with proxies.

You can keep using plex but if you have a good tech understanding jellyfin is pretty nice.

For your computer troubles I would say you can skip the pi completely if you have the space and get something like a cheap optiplex. They are dirt cheap used and can give more oomph and customization for hardware later on and are really simple to get any Linux distro on.

It's free and just as easy. You miss a few plex only features but they're not worth their money. Although I did get a lifetime license for 75$. Otherwise plex is too expensive if it's the monthly sub for the pass. Feature wise, but thats probably personal.

Generally for bigger transcoding jobs you need a legit graphics card however just modern CPUs with internal graphics can do a lot even if they are a bit slower.