Well, seeing how ASUS never released Android 9 for this phone, I'm going say that's virtually impossible.
You would basically need to port the OS yourself to the phone, ensuring it's compatible with all aspects of the phone's hardware. Since I'd imagine the demand for an OLD OS is extremely niche, you're probably not going to be able to find any developers willing to help with something that serves very little purpose (if any).
Additionally, when I first tried to type "Android" at the top of this comment, Gboard suggested "absurd"... which is a good way to describe the request. Not trying to be rude or anything, but wanting to downgrade to an older version of Android (especially one not officially released for the device) is rather foolish.
