I too went for the ApliFi system and am very pleased with it.
I currently have one Home Mesh Router and 3 x HD Mesh Points which provide good coverage throughout my "L shaped" 400 year old house with solid walls everywhere.
I am now going to extend the wi-fi to cover my detached office, in the garden, and am contemplating adding a second AmpliFi router, rather than Mesh Point, as I am told this is a better solution, but am somewhat anxious as to how to accomplish this with a wi-fi backhaul.
I have had some help in this thread
which pointed me here
I would appreciate reassurance before I take the plunge, has anyone here tried this with a wireless backhaul?