Seeing the recommendations in your Subsonic player (even the ones you don’t have in your library) currently works with last.fm auto-generated “playlists” (stations, as they call them). Streaming them is broken but easily fixable, I just haven’t gotten around to it.

YouTube links in the player would probably need updates to the OpenSubsonic specification and support on clients/servers - but I don’t really see why someone would want this if you could just stream those without leaving the player.

I’m working on a big update (multi-user support which required reworking almost everything, haha…) that’s been blocking me from doing basically anything else. After that goes live somewhere around January-February (along with updated yt-dlp which will fix streaming stuff from YouTube) one of the priorities will be implementing Bandcamp auto-search. This will allow better matching for last.fm recommendations and also matching non-library tracks for ListenBrainz.

You might be interested in Tapesonic then (a shameless plug). Streaming YouTube stuff is borked though (needs newer yt-dlp - you can try rebuilding the container yourself with a new alpine version); also grabbing non-library tracks is not supported for ListenBrainz for now (their API doesn’t return the URLs, I’ll work around this in the future; meanwhile you can still get the playlists of the stuff you already have in your library).