Higher gain antennae will only flatten the donut...
Did you try different router orientations? Different antennae positions? Be sure you leave it for more than a few minutes to see any benefits (or not).
Yes the higher gain antennas have less spread but wasn't sure what their performance is in penetration being a narrower signal. Yes I've tried different antennae positions.
Just found a management setting for giving the other router preference for router to router connection which seems to have fixed my wife's pc losing it's internet connection have to see how it goes. The setting was management section for each wifi node which you probably already know about anyway. I've given them all preference to connect to the ax88u main router. Also the ac86u the backhaul connection priority leaving the ac88u on auto, see how it goes. The ac88u still loses connections sometimes then picks it up again. Will have to give the powerline adapters a go, more money eh.