What is the best modem / router combo you recommend.

At 3500 sq feet, tou are going to either need to purchase a MESH system, like the Google   - https://www.amazon.com/Google-Wifi-System-Router-Replacement/dp/B08GG9CMLR

Or get the Cox Pano modem, and I would get at least 2, if not 3 of the Cox Pods. The downside to the pods, you have to purchase them outright and they ONLY work with the pano modem. 

Barring that, getting a power line adapter, and at least 2 Ethernet/WiFi ends, means you can use them to setup mesh networking AND, you will have an ethernet port on each of the ends, so you can use it let's same to provide ethernet to your gaming system, or TV, and then the WiFi provides mesh support as long as you have given it the same SSID and WPA key aws your main WiFi Router has. I use a tp link and it works extremely well.