Selfhosted

Like another commenter said, Mopidy can do it all in one instance. It works, but I personally find its integration with MPD clients to be a bit clunky so I don't use it all that much.

Personally I use Snapcast as an endpoint, plain MPD for local files, and navidrome for remote access to my library.

Snapcast supports Spotify endpoints, so I just switch to my Spotify stream when I want to listen to Spotify and to my MPD stream when I want to listen to local stuff.

This is more of an ecosystem than a single solution though, so it may not be what you're looking for.

If you don't mind switching to Tidal, you can do that with Plex. Otherwise, I don't think it's possible.

Maybe mopidy although I haven't used it myself. It's mainly geared toward jukebox mode (so plays on the server) but you can stream to other devices through HTTP or snapcast.

Volumio can do this with the right plugins, but it's a bit janky and not as seamless as mopidy

Try LMS (Logitech Mediaserver). It's really old but combined with plugins it's second to none.

Don’t know if it meets your ‘self-hosting’ requirements but Volumio handles local and Spotify, among other streaming services.

I don't have any experience with it, but there is a project called mopidy that I believe can play from many different services, including local music and spotify.