Maybe it's best if I just hook up some USB drives to my Pi and get my hands dirty with that. If the performance is not too bad (like you said).
Just do this and grow as you learn. If you buy WD Easystore/Elements/MyBook external drives, they can be very easily removed from their enclosures later and installed internally.
The only caveat here is that a Pi is going to be terrible for Jellyfin unless you only download media that is 100% compatible with the devices you're watching it on. If any transcoding is needed, the Pi won't keep up. A NUC or Optiplex Micro might work better here as they both have full-fledged PC hardware and aren't too much more than a Pi.