It's a stripped-down and much more secure version of android, yes. Google is optional and sandboxed on GOS.

Android with actual privacy controls: camera blocking, mic blocking, storage space definitions, google play store NOT integrated directly into the phone. Control on USB port so connected devices can be 100% ignored. 5-7years of security updates. Etc.

They're working on that with an OEM. But right now, Pixels are the only ones meeting their security features requirements.

Yes, mostly.

Are there killer features in banking apps that are not available via the bank's website? I understand that different people have different workflows, but for me, I tend to use websites over apps wherever possible.

Scotiabank works

Seems to work with USAA, though I haven't tried depositing a check yet.

USA here, it works with both my national banks and local credit union apps. One of my credit card apps gives me a pop-up on every page that it won't work because there's no Google play services, but it actually just works anyway. Had to turn exploit compatibility mode off for a couple of them though

It works for RBC in Canada.

It's not dead in the water though. After weighing up all my options, I decided buying a Pixel and immediately putting GrapheneOS on it was better for my privacy and security than getting another non-Google phone, and going through whatever processes required for whatever de-Googled Android ROM on that phone.

I don't like giving money to Google, but when I weighed up all the options this was still the best course for me. Of course, this is just my experience and reasoning.

Besides, it sounds like GrapheneOS is in talks with other OEMs so we may see support in other non-Google devices!

Take a look at the security functions the Pixel platform hardware provides. It isn't optional.