Someone finally commercialised a configuration I have been using with multiple Airport Express/Extreme routers since 2009.
From the eero
With eero, you simply plug one device into your modem. Additional eeros simply need power from a standard outlet. They automatically connect to each other to create a single wireless mesh network that covers your whole home. If you already have Ethernet wiring, you can always choose to hardwire your additional eeros.
From the article I wrote in 2009 when I first set up my Apple Airport Wireless mesh network.
Apple’s online documentation mentioned that I could use WDS to “set up two to five base stations as a unified network that shares one Internet connection.” In other words, I could setup multiple Apple WAP to create a single large wireless network that I could reach from multiple locations in the home. I could put a WAP in the basement, one in the family room, and one in the living room (near the back of the house) basically saturating my home with wireless. This is exactly what I did.
The eero doesn't support AirPlay, but from the documentation may be easier to configure than the AirPort and has seven antennas to ensure signal strength and reduce collisions. On Amazon, you can purchase a pack of three for $500. That's enough to cover a 2000-3000 square foot home and is slightly more expensive than an Apple AirPort setup.