Following up on this. . .

Been trying the X4S over the last 24 hours and there's no discernible improvement. One device marginally faster on 2.4, however, two Apple devices both noticeably slower. Range is really no different.

Hard to justify the purchase on that basis. It's going back.

What achieved an improvement was fully disconnecting the Asus (power supply and all). Then, later during testing, hooking it up again. I think the short rest did it good! Speed and range appear to have returned to where they've been in the past.

Anyway, doesn't solve the dead spot. So I'm thinking of getting a powerline with Wifi clone feature.

Another follow up:

I settled on the Asus AC86U. Bought it to compare with the R7800. While the range isn't massively different I felt it was an improvement over the Netgear on both bands.

Also since having owned the N66U I'm extremely comfortable with the interface. Smart Connect appears ATM to be beneficial (though it keeps choosing poorer 2.4Ghz channels), AiProtection too. AiMesh may be a killer feature if they bring out mini APs

I'll tinker further down the line when I expect I'll upgrade the firmware to Merlin.

The Asus RT-AC86U is a very good router but it only has 3x3 streams on the 2.4 GHz band whereas most top end routers are 4x4 on both 2.4GHz and 5GHz bands. TBH I'm surprised you didn't find the wifi range on the Netgear R7800 any good as everyone raves about the R7800 on the SNB forums even Tim Higgins himself.

Anyway the RT-AC86U isn't a bad choice at all, its especially useful for running OpenVPN on it - its been reported to give OpenVPN speeds in excess of 200 Mbps on FTTP/cable connections.

Well there's a combination of factors in play. The house is a long bungalow with thick brick walls. Realistically there's only two places for the router to go. First is by a big granite chimney (forget it!). Second, is in a home office/studio at the other far end, with another thick wall to get through before it goes anywhere.

The AC86U was just slightly better at getting through the mountains of brick! As I said margins weren't huge but the Asus' value adds won the day

I went off the idea of a standard mesh set-up since, AFAICT, none can be controlled/set-up via desktop browser. Plus, they are a little lacking when it comes to additional features.

There's a couple of issues I need to work on. Asus Smart Connect isn't super smart and I've found my Honor (Huawei) phone can get stuck with 'Network Access Denied' message on 2.4GHz due to auto channel selection. As I'm finding out Huawei phones are a little eccentric when it comes to wireless!

I've still got a borderline dead spot. Until Asus bring out a cheap(er) AP option for AiMesh, a powerline with WiFi cloning option looks the best stop gap.

Our 50-yr-old house is built in four-inch concrete block throughout, outer skin rustic brick, no dry walls or today's lightweight thermo-blocks. Even picture-hooks require an impact drill and a good bit. Wi-fi (HH2, TG582, Billion 7800NXL) just about penetrates one internal wall. Solution three years ago was a couple of Devolo mains extender units, giving full signal everywhere.

The Asus AC68U supports AiMesh and you can often pick then up for £50 second hand.
I've run one in AP mode wired to the main router for a while and its rock solid, but it should work in Wi-Fi AiMesh.

I need to have a play with AiMesh, should do backhaul over ethernet too (my preference as my AC88U is only dual band), but I need to revert my FW to Asus stock as not supported in Merlin FW.