for Jellyfin specifically, get yourself a cheap x86_64 PC and cheap or used graphics card. (or a cpu with integrated graphics)

for example, my jellyfin is running on an AMD FX-6xxx processor and a GTX 670 just fine.

the raspberry pi will work for Jellyfin as long as you don’t have any transcoding to do, but the minute you need transcoding you’ll need a GPU or good encoder/decoder chip.