Imma let you finish, but you've always been able to install vanilla OpenWrt on GL.iNet devices. I've been doing this for years across most models. Actually there are only a couple of recent GL.iNet routers that I don't own.
My rant above comes from multiple days of struggling to get a working build that doesn't have stability issues not present in the official image.
Like I mentioned, I don't recall any surprises with the MT1300 that couldn't be worked around.
https://forum.openwrt.org/t/reboot-loop-problem-at-startup-o...
https://github.com/openwrt/openwrt/issues/9273
(neither of these describe the issue i was talking about fwiw)
This post details a community firmware with images: https://forum.openwrt.org/t/gl-inet-ax1800-new-router-openwr...
That community repo is a fork of the GL.iNet build infrastructure repo, just with less definitions that allow compilation to succeed.
My understanding is that infrastructure fetches the official owrt sources and applies the patches listed in the yaml files, but I didn't audit the entire build system in 15 minutes :P