Resupply8986 Yes, flashing either the Stock OS or GrapheneOS will result in all user data being lost/wiped, but as SgtSurehand said, you can back up data with your Google account to restore it faster.
I totally understand not wanting to spend a lot of time on something just to have to undo it all. But I don't think that app compatibility should be the thing that blocks people from trying out GrapheneOS since it's not as big of a problem as some people make it out to be. It's really just a handfull of apps that either refuse to work (usually banking or other apps dealing with money or people who might try to cheat somehow) or just don't work because they're outdated. The rest work just fine.