The review part is much more concerning IMO and completely exiles FOSS from the platform (free as in the GNU definition -- you should be able to make arbitrary changes to your copy of the app without some asshole from Apple telling you that they believe your changes constitute a joke or prank [1]). The review part also re-enforces the revenue part because it costs $100/year to submit apps for EU notarization.

[1] Section 1.1.6: https://developer.apple.com/app-store/review/guidelines/

The review is very TBD for me. If they really do just check for viruses that's fine. But if it's basically the App store rules without the App store, or we get "we don't like your app" cases like Beeper than this basically ruins the point of an alternative app store.

The Apple cut is also absurd but I'm sure that will be rectified sooner rather than later.

What percentage do you think is fair to allow for a quality code review?

I think it should all be circumventable. My phone, like my computer, should be able to install whatever code I damn well please.