Yeah, it's not hard. The steps are pretty well documented. It's just not very practical. You need to do it every month and flash manually. You lose the OTA mechanism unless you also set up an update server and hack on the code to point to your update server. I don't know how well that stuff is documented. In any case, all this is extremely niche. You need a good HPC like system to have reasonable build times. Note that you are also building chromium in addition to the ROM. The parent's point was about user numbers, and I am pretty sure that that's minuscule outside of their paid users.