So I have a working IC Card on my grapheneos phone, that I now operate independently of any apps.
A key requirement for it was to activate using GoogleWallet and giving it network or phone access during activation.
Once activated, the phone can be charged on certain card charging terminals (I do the same with my watch), i.e. the ones where you just place the card on top of the reader instead of inserting it into the machine. (mostly at JR stations)
Is that what you're after?
Haven't tried MerPay or anything else.