Easier said than done, especially as there are more models that don't have any sort of support for those OSs than models that do.
This is a big hurdle. I had to buy a different phone to install a custom ROM. I bought it used, but even so still fairly expensive. And an extra cost can be a limiting factor too.
Not to mention the hassle to even enable the option to flash it and risk of bricking
GrapheneOS has a webUSB installer that is very easy to use. I would argue it eliminates the hassle. Bricking is still a risk.
Worse, banking apps may refuse to work.
I don't use banking apps on my phone. I don't like having anything installed that has access to money. I just access my financial institutions via their website. But I acknowledge they may fill some need others have that I do not. And that not everyone has access to a computer to access websites , and I can see banks trying to force app use when accessing from a mobile browser.