The day my iPhone breakes is the day I am the grapheneOS user. Switched already the battery so might take another couple of years till the update support ends
I can't speak to graphene but I'm running /e/OS on an ancient Motorola G4 Play and my (Canadian) banking app works fine. I hardly ever use the thing because I don't like mobiles and I don't have a carrier on it, but it came in handy recently when my laptop needed repair.