Graphene is actually better than most custom ROMs in that it can contain Play Services and a few other apps, effectively acting as 2 phones in one. Where I'm from, banking apps don't need strong Play Integrity, just Play Services.

I mean, carrying two phones would defeat most of the purpose anyway if one isn't degoogled in the first place, they're still getting all that tracking and they'll be able to associate it with all the online activity of your degoogled phone that's conveniently always in the same location.

Install waydroid or something or use the browser. The more you put off switching the worse it's going to get for everyone. You can also just keep another phone in your car on airplane mode and connect it to your hotspot when you need to check your account. There are ways I guess.

linux phones will never run banking apps either.