I was just saying that you can make the problem more narrow by not trying to support every device out there. Start small and pick your battles (which probably means using AOSP and using sandboxed AOSP).
I think the main issue of many previous attempts was what typically happens in the FLOSS community: there are N attempts rather than one coordinated attempt (Ubuntu Touch, Plasma Mobile, PostmarketOS, PureOS, etc.) and everybody is targeting different hardware. It's similar to how the Linux desktop got fragmented, though it's even more problematic for mobile, since the usage is probably 1/1000th of Linux desktop usage.