I like Amperfy and I think Navidrome has a Subsonic API: https://github.com/BLeeEZ/amperfy

Yes

I use markdown files + Nextcloud for synchronization. What I like about it is that I can use any markdown editor. Currently I use Nextcloup app on mobile and Pulsar or Nextcloud Web UI on desktop.

I am also on Pixelfed: https://pix.diaspodon.fr/@kevinronceray

You can find the link to join the TestFlight beta on their website: https://pixelfed.org/mobile-apps

I don’t know the current number of spots available for new testers but they post on their mastodon account when they add more.