Found it...
More fun reading, some good links at the bottom....
Addendum:
I wouldn't recommend the C7 v2 (QCA9558) for 2.4Ghz operation.
Sadly the C7 v5 uses the QCA9563 which is reliable in an AR750S (2.4Ghz mesh) but not in the C7 (proprietary driver issue I believe).
I tried meshing C7 v2 in 2.4Ghz and it would crash every ~24H which I "fixed" by using watchcat to reboot the routers every 24H - but its not ideal as it creates an interruption: the mesh would reform but AP clients would get dropped etc..
The C7 v2 seems to mesh no problem and is reliable on 5Ghz even with concurrent 5Ghz AP's. Will update if I see otherwise.
I haven't used these products but here's a link for a company selling mesh solutions that I thought looked interesting. Lots of good tips on their website.
HTH