You can certainly keep using the Netgear X4, make sure to log into it and double check the firmware is up to date. If this was my setup I would disable the wireless of the Netgear alltogether, and use the Deco for wifi. You can use 3 ethernet runs to connect the Deco pods to various areas in the home. If the coverage is sufficient there will be no need for the Netgear wireless broadcast. Is this a touch overkill? Maybe… but it will simplify the setup tremendously.

After the Deco is setup, make sure you change it from Router mode to Access Point mode to avoid double NAT, and various other issues. You will have 1 ethernet ports left on the Netgear on the main floor for anything else, and (depending on the Deco model) 1 or 2 ethernet ports at each pod to use. You can a2lays break this connection out further with a cheap gigabit unmanaged switch.

If you are a bit tech savvy you can try a similar setup with the Orbi and Eero, but will have the added burden of managing those devices with their own own app. You will also lose the graceful wireless handoff that you will get from a dedicated mesh network. In theory you can give the Orbi and Eero the same wifi name (SSID) and password, which will simplify your devices connecting to wifi as you roam around the house.

Depending on prices for additional units, you can certainly purchase more Orbi and Eero pods instead and go that route. As mentioned, you will want to pick one or the other. Personally I would choose Eero 6 for the WiFi 6 capabilities.

Hope this explanation helps!!