If there were a recommendation algorithm plugin for Jellyfin (even if it just calls out to the API of some existing external web service), that might pull me over. Until that's the case, the recommendations will keep me on Spotify

Assuming there's last.fm/listenbrainz reporting plugins for Jellyfun, then both those services will provide recommendations based on what you have listened to. Maybe not as good as Spotify's but it's something.

https://listenbrainz.org/my/recommendations

https://www.last.fm/player/station/user/{username}/recommend...