greenstumps Just download the APKs from that APK Pure website, Firefox, Ghostery and the ad blocker browsers all said the same thing.
It sounds like APKPure is doing something weird with those APKs. I just tried downloading Firefox off of APKPure and sure enough, I got an "app not installed as app isn't compatible with your phone." error.
In general, I wouldn't trust APKs from random sources like that. For apps that are in the Play Store, I would recommend using the Play Store itself if you're using or are okay with using Sandboxed Google Play, or Aurora Store if you don't have a Google account of your own.
This doesn't seem like a GrapheneOS-specific issue, but rather an issue with APKPure.
greenstumps I just hope my banking apps work, if they hate the OS then it'll be impossible to sell the iPhone.
Feel free to take a look at this community-maintained list of banking apps to see if yours is there. Keep in mind that it not being there doesn't mean it won't work. This list is not exhaustive, but rather meant to help people determine if theirs will work quickly.