I have the music app in nextcloud, which has the subsonic api, then I connect using the symfonium app in android. Works great.

Symfonium can actually connect to a bunch of different sources, including plex, jellyfin, emby, webdav and smb.

I would recommend an ultra small form factor or USSF mini pc, either dell, hp or lenovo.

You can buy these used from ebay or Facebook marketplace.

Most are core Intel core i5, anything from 6th gen is good.

They normally support up to 32GB of ram, and 1 or 2 m.2 SSDs/NVME and a sata 2.5 drive.

I would say 16GB is plenty for what you do.

Mine runs proxmox and a bunch of LXC containers, running in a docker swarm cluster, with 16GB, and I can still play with Windows VMs.

Mine idles at 13w, peaks to 35w under load.

I have a 2tb crucial ssd with my data and I do backups using kopia to an external 2.5 HD.

Really good all in one, small and efficient setup.