and NOW Apple doesn't even make them at all anymore...
That is why I mentioned that you could find AirPorts, both refurbished and used on Ebay (at bargain prices).
So he told me to go out and get a MESH router
That is an option if you don't want to spend $30-50 for another AirPort to match your current model.
There are all kinds of newer mesh WiFi systems coming out in the near future.......WiFi 6 and 802.11ax......for example. If it were me, I would hold off on a current mesh system because it will likely be behind the technology curve in just a year or two. (Mesh systems have not changed much in the last few years.)
If you decide to go the mesh route, then you won't use your current AirPort Extreme at all since you would not want to mix products from different manufacturers. Look for at least a 2 piece mesh system. With a 3,000 sq ft house, that really calls for a 3 piece system if you want better coverage throughout the house.
It's your call. Good luck!