Android Auto is fully supported and shouldn't be any more flaky than it is on the stock OS. It's often flaky due to a bad USB connection or problematic implementation in the car. That's just how it is everywhere.

Google Wallet bans using anything other than an unmodified Google Mobile Services stock OS but there are alternatives in certain regions. In Europe, there are a lot of banking apps with tap-to-pay compatible with GrapheneOS and also Curve Pay. PayPal also has a limited tap-to-pay launch in Germany.