I tried getting it to run on a Pixel 3a and struggled for hours, eventually gave up (albeit I tried running it with Wayland and Niri which seems less tried-and-true).

GrapheneOS provides drastically better usability, robustness, overall functional and app compatibility. Privacy and security are also drastically better in AOSP and especially GrapheneOS than that desktop Linux software stack ported to mobile.

All of the default apps in GrapheneOS are being rapidly overhauled or replaced. It wasn't a priority due to the incredibly good open source app ecosystem with many existing alternatives available. There isn't a similarly large and high quality open source mobile app ecosystem available for what's being promoted.

Pixel 3a will lack firmware updates regardless of what you put on it. Having serious unpatched vulnerabilities for radios and other firmware is considered acceptable for desktop operating systems but definitely not by us.

I recently installed pmos on two 3a's and it was as simple as a Lineage image. Works well too!