@Floppy Power line stuff is awful. I would actively avoid it given the experiences I've had of it. A decent mesh system with dedicated backhaul radio and well placed mesh nodes is the best bet. You could roll your own with devices which support #OpenWRT and enable 802.11s, otherwise I would go for high end #Deco devices and deal with the app/limited admin abilities.
Or potentially #Linksys#Velop, though I believe they are costly or difficult to source depending what country you are in.