I have not yet attempted Android 9, but will be doing so shortly to see if i run into the same issues.
I have been as of yet, unable to boot the android 9 image distro i have compiled from the provided sources. My device gets bast the boot image, but then is unable to find
/bin/sh and terminates the connection. My assumption is the system image is not bring written properly, if at all, as along with that specific error, I get about a million file/directory not found errors.
My first attempt was with absolutely no changes. I then noticed that my system image was listed as just about 2x the size of the output on the guide, so i did some digging. The large system size was due to the including of the google apps, so i set that flag to false, and got the system image down the 15xx that the wiki also displays. Unfortunately, this didnt make any difference as the device still fails to boot.
My next attempt is getting ahold of an EMMC module so that i can use rkfltool to flash the individual partitions and avoid the gpt image scheme, but im not sure if that will really given me much information.
Here you can find a paste of the serial output