Hello @newhome1
If you have wiring in place, I would lean towards adding a couple more Google WiFi units in locations that can be connected via Ethernet back to the LAN port on the primary Google WiFi unit you already have. This will let you place them further away from the primary (which, being located in the basement, isn't in an ideal location for a pure mesh interconnect) and will allow them to dedicate their 5GHz capacity to client traffic. In addition, use that wired network for as much other equipment as you can (any streaming video devices, gaming consoles, fixed computers, etc.). That will help preserve as much WiFi capacity as possible for mobile devices and anything that can't be wired. That's what I would do, anyway (and my home is very similar to what yours sounds like – 1990 sqft main + second plus an unfinished basement where my modem, primary Google WiFi, and core Ethernet switch are located).
