Jellyfin has come a hell of a long way since it first forked from Emby. sure its not as feature complete and polished as plex but it’s far from shit, and it’s free and open.
I run both side by side with several clients on each and have been a plexpass holder since 2013.
you can pirate on a kindle it’s just more annoying to do
Kobo is the go-to for bang for buck readers that don’t care where your files are from and have good format support. got my dad a libra 2 and it’s great, especially with the physical page turn buttons. the default reader opens most files just fine, but you can also put KoReader on them for more functionality without too much hassle.
Personally I use an older Boox Note 3 which is easier since it runs android, but is massive overkill to be used as just a reader, i use it as my main tablet and a notepad/sketchpad.