Some comments:
Afaik DivestOS had an implementation where they used gmscompat from GrapheneOS to make unprivileged MicroG work.
Generally, there are some parts of microG that work when used as a system app, for the rest you need it to be installed as a system app with signature spoofing enabled and a file declaring it elevated privileges.
About signature spoofing: I am totally not an expert so I wonder how this is done. Is it just a switch in the entire Android build that allows any app to use any signature and disable any validations? This would be horrendous! Or is it a single exception defined in a read-only part of the OS, that allowlists a specific set of signatures?
Overall I think it is very much not okay how intransparent CalyxOS was with reporting on their status. I assume the backend changes were a huge deal, but technically they could have rebased on GrapheneOS and just added other OS patches and device configs. GrapheneOS has open sourced their entire infra afaik.
I think RestlessOS and AXPOS are noteworthy too. Both are based on GrapheneOS, while RestlessOS is simply a GSI build (using the underlying outdated vendor kernel without any patches) and AXPOS adds their own patches and kernels, thus they only support a few devices.
The CalyxOS situation was kinda wild. They have a lot of loyal followers who didnt really care about security ane didnt want to switch. That was partly due to how Daniel Micay interacts with people or appears in general.