I'm using a selfhosted Joplin server instance for syncing between devices and sharing with other users. But I'm not sure if this feature depends on the server type, I would have guessed it is possible with Dropbox as well.

You actually can share notes with other users in Joplin, I use this feature a lot.

I'm afraid I don't have a solution, just wanted to say that I also have problems with subtitles when casting from Jellyfin Android to Chromecast. It works okay with external .srt-files as subtitles (I just select them in the app before starting to play the video), but scrolling or pausing the playback causes the subtitles to go out of sync, especially during transcoding. Switching between different subtitles also does not work as expected, sometimes the selected language does not match the displayed subtitle (though it does if streaming directly on the Android device or using the web client).

I even bought a Roku player because I heard it should work better than the Chromecast, but in fact it is worse (with regard to subtitles).