SourceHut actually had a really nice UI! I'll consider it. I currently don't have problems with GitLab save for the UI learning curve (and their EE is source-available) but I'll consider what you've said.
To be more concrete, we have to see if nearly all of the released games on the list have been subject to "abnormal review activity", which steam automatically excludes from the percentage
Users don't contribute builds. They contribute a specification file for how the build is made, which through the AUR is downloaded and executed. You can see the package source for every AUR package, and most AUR helpers make you look at the specification file by default.
New packages on flathub are moderated, though I haven't encountered any problems from AUR's moderation model either other than it sometimes being slow but harmful stuff is removed pretty fast