You can use most banking apps on GrapheneOS but a subset block using any alternate OS. GrapheneOS supports hardware attestation and some banking apps explicitly permit GrapheneOS via hardware attestation such as Swissquote which recently added it. Banking app compatibility on GrapheneOS is better than any other alternate OS due to some apps choosing to special case allowing it.

Google will not using their service for tap-to-pay.

Google Pay has never worked on GrapheneOS. GOS supports the attestation API -- a superset of it in fact -- but unless banking apps and Google Pay add GrapheneOS's keys specifically, they're not going to work, locked bootloader or no.

(Google Wallet runs fine for storing cards and tickets and whatnot, you just can't pay with it)