A notable mention is https://ubports.com/en/ which is different from postmarketos in a sense that ubports uses old kernels with heavy patches. That means: good support for things, but difficult future.
PostmarketOS uses the newest kernels and tries to integrate their patches into mainline kernel, so that the reliability is maintained with all kernel developers.
Yeah those are things on my mind too, especially since Google moved Android behind closed doors.
My understanding is that AOSP is still and will continue to be a thing. That’s Android. What Google has done though is put more and more new capabilities into Play Services, which are not open, rather than AOSP.
I hope someone will correct me or add better nuance though.