solid base
GNU+Linux
driver layer
Linux
and then UI layer.
Plasma mobile
The split is already there, the problem is that most Android phone manufacturers never publish the drivers (let alone make them open-source) and the only way to get anything but stock image running is to just rip parts out of the stock image, which significantly limits what you can put below it (i.e. Linux version) and on top of it (i.e Android Java gubbins). And you can't "just replace AOSP", as it's a huge complicated thing (kind of by design) which allows vendors to tightly couple the drivers to the system image. The idea of all these "mobile Linux-es" is to get rid of AOSP entirely, replacing it with "desktop Linux userspace" (systemd, musl, D-BUS, NetworkManager, pipewire, upower, mpris, libnotify, Qt/GTK, Plasma/Gnome, etc etc etc). A DE is an integral part of this; you can't build and run Nova launcher just with Wayland and Pipewire but without Dalvik and Android SDK/NDK, and remaking all of that from scratch would be an insanely hard undertaking.
To put it another way,
Get a good base that is removed from Google, THEN do this project.
This project is required if you want to make a "good base", otherwise that "good base" would just be an empty TTY that you can't interact with because there's no on-screen keyboard; besides, that "base" is already there and has been for 20 years, what's missing is the drivers.