LineageOS is based on AOSP and works well. I don't understand the banking app thing either. I suspect it's a regional issue. I can log in to my credit union account via any browser, and if something needs MFA it should be able to use TOTP which works on anything.

Android in practice is full of proprietary blobs, stuck on old kernel versions, and the hardware is barely supported. Lots of downstream crap from the vendors not playing nice. Most devices running Android are instantly doomed to be e-waste. You can look through devices postmarketOS supports, and anything without mainline kernel support and most stuff working is basically e-waste unless someone puts in a lot of work for that particular device. It's a little bit like how modern GPUs don't work without blobs in the kernel anymore and you have to go back to Haswell era or older for things to work with all free software, but the state of smartphones is a few steps worse than that due to their locked down nature.

Pretty much any OnePlus device (other than ones still too new) seems to be a good bet for decent software support (both LineageOS and pmOS). Though annoyingly stuff like the 3G shutdown makes a lot of the earlier models unusable as actual phones these days. At least they can still be computers. Not quite e-waste.

Yes we have banking websites but they are increasingly moving to an auth model where you have to enter an otp generated in the app but the app refuses to work on non-verified devices.