...and as I've posted elsewhere, my 2926ac mesh root can "discover" and add an AP903 in the garage to its mesh, but it cannot "discover" an AP903 in the kitchen, which is about 5m closer, without the two intervening walls and which apparently has a much stronger RSSI according to the "AP Discovery" function

I have no idea how this can be.

I strongly suggest that you get your "mesh" working in close proximity before you start spending....