Nice. This brings back some memories... I had a fun time with rolling my own builds of calyxos with Android auto stubs as system apks so it worked. I made the mistake of signing them myself, locking the boot loader, and having a failed update knock my phone into a bootloop. Somehow managed to fix it by breaking it even more but the whole experience involved a lot of learning and wishing calyxos had a variant with the stubs already included.