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.
The thing is, I don’t really care about Android apps, and honestly supporting them probably adds a bunch of limitations since they have a lot of expectations on the system.
I just want an immutable base system w/ flatpaks, a basic dialer, a robust SMS/MMS app, Firefox, and good enough battery life (15 hours w/ moderate screen on time). Basically, openSUSE Aeon or Fedora Silverblue with phone-specific apps.
I’m happy to help port the various software I want to use, but I need the phone to work as a phone first.