I have not done this, but I have done a similar thing for a different router with a different fork of OpenWRT (from easywrt.com), which helped me to get the real OpenWRT on in 2 flashes.
I just installed X-WRT over untouched stock firmware on AC-RT58U, then just used the LuCI GUI to upload the OpenWRT sysupgrade .bin. So far so good, seems to work fine. I'm a newbie so I don't know if I have to mess around with jffs2 volumes as described in the OpenWRT documentation: https://openwrt.org/toh/asus/rt-ac58u If anyone could help me out, I'd appreciate it!
Has anyone installed 19.07.4 update on asus RT-AC58U ???
I downloaded the image https://openwrt.org/toh/asus/rt-ac58u for updating from version 19.07.3 through the web interface of the router. After the reboot, the version remained the same.