Differences between firmware, GUI & OpenWrt (?)

Yes and no. The GL GUI uses wrappers to modify the config files - so it's not like luci itself, which highly utilize the config files; it's more like a mixture. Not all settings (and changes) will be shown in the GL GUI.

luci is a 3rd party tool - so that's why no support can be offered.

Feel free to scroll through 2200 posts: Flint 2 (GL-MT6000 ) - bug reports - collective thread

From my feeling: Doesn't matter if you are a normal user with not pretty old devices.