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.
You can keep your old phone around for it but they should solve the problem. Motorola can likely help us with getting it resolved once things are further along.
