@OzarkEdge thanks for your response. The house is wired with ethernet and not adding anymore than what exists. So I guess I could technically add one router to the first floor but then not sure how I would connect that back to all the wired connections which terminate back in the basement without running another cable. The first and second floor already have Google Wifi pucks wired to the main puck in the basement.
Given existing Ethernet to support wired backhauls, your new equipment does not require a third band for dedicated wireless backhauls, so you can avoid tri-band routers that dice up the 5.0 band (5-1, 5-2)... a WiFi6e or tri-band purchase would be just to add the 6.0 band for client use... not terribly necessary (but 6.0's 160MHz bandwidth without DFS disruption would be nice, if you have any WiFi6e clients)... your choice/expense.
If one router at first will do it, you would plan to locate it centrally on the middle floor, if possible. If two routers are determined necessary, you would then want to spread them far and wide... 750sf is not wide enough imo, so perhaps 2nd floor and basement, diagonally. You would want to plan for these variations.
As for the routing of existing Ethernet... they alway seem to forget the second cable where you need it. Consider your options/workarounds. One is to put a router (no AP) in the basement and AP(s) at the other end of the cable(s).
If you had the ASUS router, you could trial its coverage to answer the question, 'is one enough'. I've used the AC86U, AX86U, and now the AX86U Pro to cover my three levels... router on the middle/ground level at the end farthest from a detached garage where my AiMesh node is... 77' but through interior drywalls/kitchen and two exterior brick walls... this stretched the wireless backhaul when I had it, but roaming is good... you want clients to have a clear choice of the better signal, not two strong signals everywhere.
If using more APs, then using less transmit power may be in order... maybe your existing Google APs are not too powerful. Have they presented any particular WiFi issues in your space?
OE