I’d be happy to use any VPN client that can talk to an OpenVPN server. Doesn’t have to be any particular client. I’m not familiar with wireguard app but am open to trying it.
I’m impressed you got Android Auto working (presumably wirelessly?) with an active VPN running. I totally failed in that regard and everything I’ve read says it’s just not possible. I was able to do it with a hardwired connection, but AA permissions have to be set just right.