@Scoutbob the “Bump in the night belt, do you have one?” thread is also pretty useful and sounds like there is some overlap with what you want to do. If SHTF you just don your Bump In The Night Belt (BITNB) and off you go. Presumably you will have trained with it, so all the stuff is where you remember, and it should be pretty minimalist.
I’m sure there’s a use-case for mag couplers, I think if you “know” you are about to get into a firefight having an extra mag right there for use that is maybe a little faster reload? I personally would rather just have spare mags on my person than having the weight on the rifle. If I really want a lot of ammo without a reload I think I’d go with @Jeff-A1’s PMAG drum.