I too, just purchased a GL-AR750S-EXT (a.k.a. Slate), and am about to undertake the same path to install OpenWRT (well, one from here, as I understand there is a GL.iNet version of OpenWRT already installed on the device). It took a while to wrap my head around this thread and references to other pages from this thread, but I believe I can conclude the following:
Based on this thread (please correct me if I'm wrong), it appears I have a couple different options for installing OpenWRT on the GL-AR750S-EXT.
Option 1: go with the "Current stable series" (19.07+) of OpenWRT
-or-
Option 2: live life on the edge, with the "Next stable series" (21.02+)
Among other differences, sounds like 19.07+ only supports running out of NOR flash, but that 21.02+, allows using NOR flash and NAND flash.
Sounds like either path is best accomplished using the uboot method (power down, ensure only 1 network cable is plugged in, hold the reset button, power on, wait until the led blinks 5 times and stays on, then release reset button, etc...), detailed a bit more on the "oem_easy_installation" web page referenced above.
If I go with Option 1, sounds like I will use the "vanilla" (a.k.a. "-generic-") binary image. "16F84" seemed to suggest that to get to the latest build on the AR750S (19.07.8), I should first use the uboot method to install 19.07.7 (to avoid the unsupported format warning message/installation failure), and then upgrade through LuCI to get to 19.07.8.
If I go with Option 2, sounds like I will use the uboot method to first install the "-nor-squashfs" (NOR only, but "NAND aware") binary image, and then use LuCI to upgrade to the "-nor-nand-squashfs" binary image (and any/all subsequent upgrades). Though having said that, "memilanuk" expressed reservations with the 21.02+ path at this time, though I think because package upgrades were applied, that probably shouldn't have been. Installing "-nor-squashfs" followed by LuCI-based upgrading to "-nor-nand-squashfs-" without any further updates found on the 'Software' page, may be ok.
Regardless of which path I take, the "firmware selector" (16f84 provided a link, above) has the various images, based on which release I wish to move forward with.
Pleased to report that I did Option 2 - as "16F84" indicated, went from out of the box to 21.02 in less than 15 minutes, with no issues.
Many thanks to memilanuk and Budoka for blazing the trail, 16F84 for giving me the courage to just go for 21.02+, and bobafetthotmail for keeping us between the lines!
JohnG