XR500 and using a second netgear router as an access point to expand coverage

I would most definitely get one of the extenders over a extra router.

https://www.netgear.com/landings/mesh-wifi-extenders/

You really want the mesh functionality, this will insure your wifi devices automatically finds and uses the best signal and automatically switches between stations , If you add a second router your wifi device first needs to disconnect from the first router and then reconnect to the second router, this will result in hiccups in services. (skype call disconnected, download failure, ......)

I myself am a very happy customer of a Orbi System so I'm no Nighthawk expert and there is a very very tiny chance that maybe you can add a second nighthawk router as a mesh point but someone else will need to confirm that.

I have both professionally and personally spend a ridiculous amount of money on wifi systems over the past 20 years and I can confidently say

"Once you go mesh, you will never go back"

Cheers,