Nighthawk D7800 and mesh

I'm deliberately keeping this simple:

For reasons far too convoluted to go into here, including UK Listed building restrictions, I have a network that combines a D7800, a couple of S8000 switches and Powerline adapters with Wifi enabled.

All Netgear devices are on the latest firmwares where possible.

The D7800 WiFi is not strong enough to cope on its own, hence the current interim arrangement with Powerline WiFi.

We are purchasing the property next door which has some hardwired connections of unknown quality, (possibly cat-3 installed before the planning regulations changed in the mid 2000s), and the planning officer is not sure if I'm allowed to open up the walls to replace the cables, so I want to take the opportunity to tidy the network up with better WiFi for now.

I really want to disable the Powerline Wifi and extend the Netgear into ideally a mesh of somekind. Total size of the property is 2200 sq feet.

There's nothing wrong with the router, so I would rather not throw it away.

Options appear to be:

1. Turn the router into a dumb modem and spend a fortune on a new mesh, which don't really seem to scale down to a relatively small 2000 square foot

2. Go old-school and buy a half decent netgear range extender and forget mesh

I'd like an option that supports a hardline backhaul if possible, just in case we get permission to change the hardline cables, or just a dedicated backhaul channel.

Any suggestions based around keeping the D7800 as a modem/router?

Anyone any good experience with the Wifi Extenders?