The easiest thing to do would be to put your VPN provider’s conf into the WG app on your phone & leave it as a connected device but not using the WG Client of the GL device/router. (GUI → VPN → VPN Dashboard → VPN Client → Global Proxy → Based on the Client Device). It would default to blocking your phone from accessing any of your LAN-side computers however.
Technically speaking what you describe can be done: in reality there is no ‘server/master’ paradigm using WireGuard. It’s all peer to peer. I agree w/ GL that it’s far easier to grasp client/server though. I’m highly confident they wouldn’t support/assist setting up such a custom solution. You’d be better off directly asking on the OpenWrt forum. OpenWrt Linux is what GL builds their GUI upon.
Regardless you should make a backup of your GL device before diving deep into configuration files. You can always bail out with a backup. See