There are custom ROMs in which you can use banking apps, UPI etc. And they are stable.
You know Android is open souce, right? Developers use google's source code and make custom version of Android and release. There are lot of custom ROMs available, these are some popular ones. Lineage, Pixel Experience, Havoc, Crdroid. Since you sound new to custom ROMs, better take time to understand fully before attempting. Google for more and check out xda forum. You can find your device channel in tel eg ram, join there.