For me I’d just say oh well, gotta fix it when I’m home again.

Otherwise I’d probably write some script on the server, which reboots my router when the server either doesn’t have internet anymore or can’t ping itself.

AFAIK you can enable the snapcast server and then use an app like SnapCast to stream to your mobile devices.

That’s incorrect, Graphene OS has Android Auto support.

You could try getting a Raspberry Pi Zero together with some kind of SPDIF output card, but that will probably go over $30.

I have no idea what pricing is like, but you could possibly try getting a used Logitech Squeezebox player.

If you’re desperate to stay on the cheap and don’t mind BT quality, you could also install Snapcast on an old phone, enable the Snapcast player provider and then use the phone to connect to your speakers over Bluetooth.