Regarding Google Wallet, you are right, Google Wallet blocks NFC payments. But many banks' apps implement Android mobile payments to be able to pay too. You can check if any in your country do by installing all the bank and credit card apps and without needing to sign up or log in, you can go to GrapheneOS's settings, connected devices, NFC, contactless payments, default payment app,and you'll see a list of all the apps that implement the feature. Then you can choose which of those banks or credit cards you want to sign up for to use for mobile payments.
Monzo app works according to this report. You can check this community driven page for a list of compatibility reports for many bank apps in GrapheneOS.
Battery life is great on my Pixel 8 with GrapheneOS. I charge the phone every 2 days, but I don't use the phone that much.
Android Auto works fine and all the apps that used to work in the stock Android Auto that I've tried also work fine in GrapheneOS Android Auto.
You can install the Pixel camera app from the play store to get the same picture quality as in stock. The GrapheneOS camera app has a different look to the stock pixel camera, but is also very good. GrapheneOS's camera looks more natural.
It is as easy to use as stock OS, except that you'll need to install some apps that would come pre installed in stock. But it's very flexible, so you can make it harder to use too if you want to avoid installing Google apps, want to revoke as many permissions as possible to the apps, want to use profiles etc. But you can simply ignore those features exist and use your phone as you would it with stock. But unlike with stock, you'll always have the option to fine tune the system to your liking if you feel like it.
Updates are over the air, and are set up to auto-install after X hours of inactivity, so they are very hands-off. If you stay within the stable channel, which is the default, you'll virtually never encounter a bug. In the very rare occasion a bug goes unnoticed in the testing phase and ends up in the stable channel, the team is quick to fix them or worst case revert the change that introduced the bug. The bugs you would be more likely to encounter, although I haven't encountered any, are with third party apps that have memory management bugs that either memory tagging or the hardened malloc end up killing the app to avoid you being exposed to those vulnerabilities. For those cases, there's a compatibility toggle in the settings, but I've never had to use it.
In summary, there shouldn't be much of a problem. Specially if you keep it simple to a single profile, install Google Play Services, and any Google app you might like. Then you can gradually tighten the privacy of you phone as you get more comfortable with the system.