AiMesh will run one router in normal mode and the other in AP mode(sort of), but you will not access that AP node separately, it will be managed by the Main router once Aimesh is set up. I use a AC3100 as the main unit and an older AC68U as the AP. Note that the T-mobile version of the AC-68...
Not/never had any crashes with AC68U & an AC3100 mesh setup. But had crashes a couple of years ago while using an AC68U as my main router. They run very hot. Try cooling, better heat sinks, etc. Move it or raise and add a fan blowing on it and see what happens. That all helped but I had to...
Have you looked at the supported routers? The AC66U is not on the list. If you hacked it to accept a different firmware that is unlikely to work. Hacked versions of the TM-AC1900 (T-Mobile) used to work. Recent firmwares do not anymore, even though this router is a modified AC68U.