What one programmer can do in one month two can do in two months. Mullvad is small and dedicated enough to not have to wait for a separate QA team, etc.
Create a user-defined collection (like group of favourite movies)
Collections exist along with ratings.
Adding user-defined tags
I think you have to be admin to do that bevause it's part of the library metadata. Shared libraries would have to use playlists or collections which are per user. I don't use Jellyfin so I'm not sure what you're referring to.
Filtering by tags or any meaningful filtering
Filters work well. Similar to iTunes. The filters aren't listed in alphabetical order for some fucking reason, though.
Filtering by languages (however this could be done by using combination of 2 different plugins that add tags and create collections)
I think you can filter by movie language? Never done this. I don't think it cares about the languages available in the audio streams.
[Important] No movie/show relations (e.g. The Lord of the Rings 1 -> 2 -> 3)
None on Plex.
No way to know if movie/show belongs to any collection
There's automatic collections but there isn't a UI element that shows if a movie is missing from a collection or vice/versa.
No it's worse. Old accounts are grandfathered in to downloads when a library is shared with another user. Otherwise the library recipient needs a plex pass too.
50-100TB