I an not sure of you are refereeing to the issue of 2.4 GHz conflict with USB 3.0, or of there is another thing.
If it's about that, then at least some of the issue isn't particular to this device. See this
if you experience it, you force the device to work as USB 2.0 by connecting it via USB 2.0 cable, or you could try a long USB 3.0 cable to Phoebe the device further from the router (or a better shielded one) in order to reduce interference. There are some other tips here and there on the forum.
This topic also has some useful posts. Xiaomi Wifi Router 3G - 18.06.x / Wifi issues 2.4GHz + 5GHz.
As for the WiFi signal, you could just put the two router in the same location (preferably different channels), install a software to measure WiFi signal (on laptop or mobile) and go around the house to see which one works better.
You could try the Archer C7 as the main router, sacrificing the slight difference in the CPU, and hoping that the RAM will be enough for your use, but depending on what packages you install, you may run out of FLA space. Archer C7 had only 16 MB while Mi R3G has 128 MB.