> Will the sandboxed google play permit banking apps to work using TPM and secured credentials?

Apps that don't work don't fail due to technical reasons but because upstream says so, i.e. Google Wallet. My banking app works just fine.

> I would expect whatever you initialised before grapheneOS is wiped before you can run the alternate OS.

Yes.

> Is termux possible with a root/sudo function?

GOS doesn't support root by itself since they deem it a security risk, but it's possible.