I believe F-Droid signs the packages it distributes so that creates a painful choke point. Revoke F-Droid's key and it will break all of F-Droid instantaneously. The only exception for F-Droid's signing is if the build is reproducible, which is a high bar for a lot of projects, and then F-Droid will use the upstream signature.
Also, they're trying to close the ADB loophole.
Plain AOSP is already pretty brutal. An alternate OS is practically a non-starter. Phones aren't just web browsers and SMS.
Not to mention that the camera is going to suuuuuuuuck.
Forking or improving AOSP is more viable but none of the more mainstream ROMs want to piss off Google. That's why most LineageOS forums forbid talking about defeating Play Integrity.