My XR500 is on the ground floor of a three story terraced house, 120 years old with some thick walls. Even so, I did experience generally good WiFi connectivity for general web browsing across the house.
I only tested one EX extender but the Netgear documentation suggest they could be daisy chained in bridge 'extender' mode but clearly each hop will add additional latency.
Deploying in AP mode would be result in a star configuration but you'd need to run LAN cables to each EX and if you could do then I be cabling PC/Consoles directly if that was the case.
Whilst 5Ghz offers less interfernecne it's not so good penetrating thick walls. 2.4Ghz can be better in older houses but is much more prone to interfere from neighbouring Wi-Fi networks and other devices which operate on the same frequency bands within the home (e.g. cordless phones, microwaves, bluetooth, etc).
Good to specify the frequency/channel manually in my opinion, when I left the XR500 Smart set-up it put me on 2.4Ghz network with 6 competing networks in my neighbourhood with spectrum conflicts. I have manually configured an uncontested 5Ghz band (DFS 100) which was the goldilocks zone for me.
Would you say then that 1 unit was enough coverage. I have 3 stories with the router in the basement - no choice. Do these daisy chain or just star topology? I had bad experience with the EX7500 extender which just could not maintain a router connection with the XR500 on the main floor - worse on the 2nd floor with the second extender. The X6S router did better at maintaining the router connection to the EX7500's but still intermittent. I didn't play with the channel settings though and I had smart connect enabled. Using Orbi now for the WiFi with the XR500 wifi radio disabled but would prefer to extend the WiFi of the XR500 using extenders. Thanks.