So I attempted to upgrade SNAPSHOT r0-679200d, using 23.05.3 firmware by following Upgrading OpenWrt firmware using CLI instructions.
First attempt failed to update the firmware.
I ran service wpad stop
and sysupgrade
succeeded, however router soft bricked (solid orange light).
I was able to recover, by following the TFTP recovery debricking instructions on the Xiaomi Mi AIoT Router AX3600 wiki page.
After restoring the stock firmware and enabling ssh, I was able to successfully install OpenWrt.
I wonder if the problem is that the build was not using a 'unified rootfs'.
When I upgrade the second router, I will try this suggestion: