I think most banking apps already do work on GrapheneOS (not sure about TPM/secured credentials though). Graphene IIRC keeps a compatibility list somewhere. Some don't work, of course, but more do than I would have expected.

For me, the big question is if Google Wallet & its NFC payments will work. They don't on GrapheneOS currently, but if Motorola plans for this to be a fully Google-certified phone with GApps and everything, it will have to, somehow.