Depends on what you're trying to run on the gateway and how much wifi you want to deliver.
First off is cabling. If you can create even a partial wired backbone, do so. Ethernet first, MoCa over coaxial second. This will provide much more reliable and higher-bandwidth backhaul, and client links as well, regardless of if you go with a whole-house consumer product like ZenWifi, or a wire-first centralized AP system like UniFi or Omada.
Regarding AX, I'd hold off and go AC Wave 2, as there's minimal benefits in the real-world right now, if any, plus all the AX hardware and drivers are pre draft 1, so who knows how compatible and/or patchable they will once draft 1 spec gets adopted and enabled.
On the gateway, again, totally dependent on your needs and taste. If the baked-in features and stability get you by, great. If they fall short, you can always (hopefully) put the system into AP/bridge mode (effectively running as just a wireless AP system at that point), and drop in more solid discrete routing (Ubiquiti, Mikrotik, pfSense) and switching if needed (Cisco SG, HPE, etc.).