Masraenki Probably the most secure Android OS aside from GrapheneOS is Google's, and I suspect apps that want a known platform will be satisfied by it. So you might consider a used Pixel 6a or newer running Google's OS. I suspect some people would suggest Samsung... I think newer Samsung devices get frequent security updates, but I think I read that updates are slower for older devices, which might be an issue for banking apps. And iPhones are reasonable in terms of security and have reasonable support lifetimes.
Just my personal non-expert opinions!