UK Banks: HSBC, Lloyds, Starling, Monzo, etc.
Are you asking whether you should use grapheneos, your fitbit or a second phone for banking?
I think OP wonders if GPay on Fitbit works on GrapheneOS with his bank, or if he needs another phone to set it up.
A bank card is far more practical than a second phone. Even if Google Pay did work on GrapheneOS, I would not use it. It looks like a privacy nightmare.
Don’t need them. Halifax has online banking in a browser. When they merged with Lloyd’s, they moved systems so I’m assuming it’s the same for Lloyd’s.
Far too slow moving money between accounts while standing at checkout with a queue of people waiting behind you.
If you must, the least invasive of the wearables that don’t suck is Garmin. Get a Garmin watch that supports it.
Paying with NFC does lower the risk of getting a card cloned. The payment generates a unique code that’s worth only what you paid.
If you want a device to do NFC payments you’ll need to look somewhere other than GrapheneOS. (Believe me, I’ve tried everything)
Garmin pay, assuming you bank supports it