I'll list some cans and cannots.
- NFC payments can be done by Curve, Garmin Pay (needs Garmin Watch), or your banks own NFC payment system if it has it.
- You can still use assistant, just not via Hey Google, this also applies in Android Auto, it just needs the button to be pressed.
- Call assistance is mostly handled via the Dialer app, which can be installed on GOS, I use it for the spam protection.
- Now playing for music wont work, there are 3rd party projects, but they require some work arounds which use ADB.
- Good backups, Seedvault is no where near as good as Googles.
- You can still use GCAM, which will do a lot of Pixels camera stuff.
- Think Video boost is a google photos feature, or GCAM, both can be installed on GOS and work fine.
You will miss out on a lot of the AI crap, circle to search wont work for example.
Make sure key apps work like banking first, just ask on here, I am sure someone will have used the same bank, mine flags as an "insecure device", this is because GOS cant pass specific safety checks, only Google can allow an OS to pass, but it doesn't mean less secure, its just like shitty DRM.
How degoogled you want to be is entirely up to you, thats the benefit of GOS, cut out what you dont want, keep what you do!