Hi Ryan
Thank you for your asking. Due to your firmware is now WW-15.1618.1810.77, please open your "Auto-download system update" and auto update to WW-15.1630.1907.98 by the following steps: Settings>System>System updates>3 points at the top-right row>Auto-download system update
Then perform manual update to WW-16.0410.1910.91, and auto update to the Android 9 system you prefer. e,g. WW-16.0420.2003.24.
***If you desire to cross over the Android system, you will need to perform manual update, if you simply want to update your firmware system in the same Android, then auto update is a easier way to perform. Please note that once the update is completed, all data, apps, settings and personal information will be deleted. Kindly backup your data first.***
#Manual Update Guide
https://www.asus.com/support/FAQ/1009748/#ZS600KL Firmware Spot
https://www.asus.com/Phone/ROG-Phone/HelpDesk_BIOS/