I do not know for GL-AR750S, but my AR300 is pretty fine of stock OpenWRT.
The NOR and NAND issue was an issue with older versions of OpenWRT as OpenWRT did not support one of those memory types (I believe NAND). Nowadays you have 2 firmware images available for this router. One for NOR and one for NAND. So that’s not an issue any more. The only slightly tricky thing might be switching between NOR and NAND firmwares. There are commands for that.
I believe you could simply install it from luci or uboot. Do make sure that if you’re running on NAND-firmware (default) that you install the NAND image of OpenWRT (same for NOR of course). Do not keep the settings when going from factory firmware to stock OpenWRT.