ET12 support is unlikely to happen at this time.
I added XT12 support because I wanted to have two different devices based on the new HND 5.04 SDK at the time, and also to test the waters with a Zen Wifi device.
The XT12 barely has any users. Roughly 80 downloads on the week the firmware was released, versus over 200 for the GT-AX6000 released at the same time. The RT-AX86U gets 3000-4000 downloads on a new release week. That's simply nowhere popular enough to justify the added workload, so for now, I am not going to support the ET12.
One of the problems with the XT12 and ET12 is the lack of USB port, which greatly reduces the usefulness of running Asuswrt-Merlin: it means no Entware, and most addons won't work either. So even if these routers sold more, there would be a less compelling incentive in running Asuswrt-Merlin. People specifically wanting to run it would rather go for a model with USB ports, to allow them to use addons such as Diversion.