With regards to Spotify, assuming you are able to build a reasonable library by other means, I’ve been massively impressed by Navidrome.

Run it through a free Tailscale tailnet, and you can access it wherever you like (as long as the accessing device is in your tailnet). Run it through a reverse proxy on a VPS and you don’t even need your accessing device to be in the tailnet.

So I now have Symfonium on my phone, and Feishin on my desktops, all accessing the same library.

The only part of that that has to cost any money (aside from buying music, that is), is the £5 I paid for Symfonium.