WiFi Router + Extender vs Orbi Mesh Wifi

We are in Canberra Australia and have a VDSL2 connection with iiNet. I have a Nighthawk X4S AC2600 Wifi Router, configured and functioning as a modem router to avoid the need to use a TG789 modem supplied by the ISP. This hasn't device missed a beat and been dependable since I bought it.

Nonetheless I got the technology the itch we all get from time to time, to look at whether it's time to upgrade technology while it's hopefully not too later to sell the X4S. What started the itch? All the recent hype about new and improved Wifi 6 devices and the more recent Orbi Mesh Home Wifi such as the RBK852.

Presently the Nighthawk X4S AC2600 Wifi Router is centrally located nearest Hi-Fi, main TV and where we use our Android phones and Windows 10 notebooks most. At one end of the house nearest my studio, we have EX7700 range extender, which I don't understand well but have configured to use the same SSID as the router. This seems better than configuring it to use different SSID's. It seems that the devices closest to the extender did not willingly or easily connect to it. With one SSID the same devices seem connect and remain connected better.

Therein I suppose lies my first question of many, regarding our current setup versus what could upgrade to. After reading a little but not yet a lot about Mesh Wifi, how, with regard to having one SSID, is our current setup different to an Orbi Mesh All-Of-Home WiFi? If there is little difference or the difference are only symantics, will it ultimately, be possible to use a second range extender to provide and increase coverage at the other bedroom end of the home? Perhaps this time an EX7500 range extender also using the same SSID. This would be the cheapest and possibly easiest way to extend the life of our current technology.

Next. The Nighthawk X4S AC2600 Wifi Router seems to be NetGear's flagship modem router. Do any of the new lineup of AX or Wifi 6 routers also function as modem routers?

Finally, none of us are gamers. So am I correct to understand that the new AX WiFi 6 routers, extenders or mesh wifi, are of little if any, benefit in terms of speed, unless the client devices are also using the new AX or WiFi 6 standards? If so, and given we are unlikely to upgrade devices to AX or Wifi 6 compatible ones in the short to medium term, is it in terms of speed, pointless to upgrade to new AX or Wifi 6 routers and extenders, or Orbi Mesh HiFi systems?

What about in coverage? Does AX or Wifi 6 bring improvements in terms of wider coverage?

Given all of the above might the best and cheapest solution for our situation, simply be a second range extender at least until such time as AX or WIFI 6 becomes more widespread? Am I on the money?