The migration from ar71xx to ath79 could indeed be the issue. In fact, I once installed 19.07 on my GL-X750 following the device specific openwrt installation comment (telling to use u-boot indeed to go from ar71xx to ath79). But, the GL-AR750S device specific openwrt installation does not have such comment. It says: use web-gui (as in git commit). Nevertheless it is worth trying. I will do so and keep people informed. If success, I can update the device specific techdata page.
I installed the 21.02.0-rc2 nor (not nor-nand) on the GL-AR750S using u-boot web recovery. This nor file is an u-boot image as opposed to the nor-nand file which is a tar archive. Note that GL-X750 ath79 upgrade files are also u-boot images. It all makes sense. The box runs fine as access point with very satisfying performance. I have not bothered retrying the nor-nand flash anymore. Basically I am happy and will buy several GL-AR750S to replace all my existing access points.