Selfhosted

I know you've mentioned it, but Navidrome is probably the best choice, but it won't be exactly what you want since you need to interact with a proprietary service.

But, that said, I've gone through basically every single music server I've found and ended up landing on none of them.

They're all broken or missing features that another one has, and there's no One True Music Streaming Server, just a bunch of mostly-kinda-sorta-almosts.

At this point, I just use a network mapped directory and/or a synced copy on the sd card of my phone and local players and don't bother with anything more complex anymore.

The local players that can play media seem to have a much better, richer feature set than ANY streaming one does.

You can use Subtify with Navidrome to import your playlists. It matches your Spotify playlists with your local music.

I have tried them all, it's Navidrome. It's actually less resources for me to run 4 instances of Navidrome for my different users than it is to use any of the other servers that allow for separate user libraries. Just make sure you alter the folder scan interval, it's stupidly frequent by default.

Agreed that there's no all-in-one solution to play local music and music on Spotify (if I'm a premium user). I vaguely recall there's a solution to automate playing Spotify music and record it in real-time (since you cannot download music directly) but it seemed too troublesome, so I eventually chose spot-dl4 to download music from YouTube using Spotify playlist, then the folder got imported into Lidarr/Navidrome, then my Symfonium on Android connects to Navidrome to get the songs.

It's quite a bit of manual work to add songs to a separate playlist if I like something on Spotify then use spot-dl4 to do the download. At least, I successfully keep a copy of my favourite songs on my server.