I learned about the Halium project half a year ago. It is a middle ground for Linux userland and the Android kernel to communicate. It doesn't try to get rid of Android if I understood it correctly. Instead, it tries to redirect low-level calls to the existing HAL.
Working on Halium is a great opportunity to learn the Linux kernel and I want to spend time on it after Android 9 gains support.
Halium targets CyanogenMod/LineageOS as the default codebase. It is probably because it is the most successful ROM and no major company uses the AOSP device tree for their modified versions of Android.
The official website covers most of the things you need for getting started and you can join the official channels in case you need help.