I do and did this with all my previous phones. Unfortunately, it comes with so many obstacles nowadays like non-working banking apps, that I'm unsure how the future of custom roms looks like.
If I need to buy a new phone, I'll pick one with good custom rom support and a cheap second one just for the banking stuff.
I don't understand why running a proprietary banking app is a deal breaker. It will never work and should never work for security reasons. The bank wants you device to be controlled and locked down by google so that you don't get robbed if you lose your device. Putting your finances on a device with weak encryption and no locked bootloader is a bad plan as far as I'm concerned.
I know many people like continence but for me freedom is more important.