Adding third satellite for better wifi signal

I have a home that was doubled in size a few years ago, 4000sqft total, with some exterior walls now interior,  I have had several mesh systems and purchased the w121 system for its 5000sq ft coverage, so far I barely get good signal in my outer rooms, doorbell cams, and cams outside, my question,  can I add a 3rd max pro router as the main and use my 2  max w121 as the mesh system? I know I can add a 3rd w21 to the system but was thinking if I got the better max pro as the main router, I may get some better service in the main room and get longer signals. Or should I just get a new system all together? I've had the Google mesh, Samsung mesh and some linksys (older gen) that I didn't have too much luck with.