I feel this comment so much. Like you, I’m a Symfonium user, after reluctantly admitting that DSub has stagnated. I really wish someone would just assume that mantle.
Tempo is probably the best of the current FOSS subsonic-compatible apps, and it ticks most of the boxes, but it still has a way to go to become my daily music app. I posted a review/wishlist in its GitHub discussions for what it could implement to be great in my view, but development seems really slow for it. It might be just one guy, I’m not sure.