Quite a few banking apps work on Graphene OS. My bank isn't listed and it works on Graphene too (I really should contribute to that list). The big downside is probably having to buy a new phone for it but if you need to upgrade anyway it might be worth considering. The only app I've run into so far that didn't just work was the Target app but it works with a bit of tinkering. I've only been using Graphene on my phone for two weeks now but all of the apps I used on my old phone are on here and working.