My ISP (Ziply) allows for 2 DHCP leases at a time at the ONT, this is extremely useful when swapping out a router. I ran two routers for a while without issues. They do this on purpose but they still control the bandwidth allocation at the ONT and I don’t get twice the speed by using 2 routers.
If you have the original Google WiFi pucks, I have recently flashed them with OpenWrt and it has been great.
Took a bit of time to fully set them up properly, got locked out and had to reset them multiple times. But am super happy with what I have now. Running multiple VLANs, a real firewall and the WiFi seems much better.