If you download diagnostics (three dot menu on the ZHA integration page), what does the energy scan section look like? It will be towards the end of the file and look something like this:
"energy_scan": {
"11": 9.713248103580147,
"12": 6.789392891308996,
"13": 73.50699819621309,
"14": 84.164247274957,
"15": 88.70042934643088,
"16": 80.38447947821754,
"17": 75.96022321405563,
"18": 73.50699819621309,
"19": 9.713248103580147,
"20": 73.50699819621309,
"21": 3.2311094587038967,
"22": 4.69985354430736,
"23": 3.6632469452765037,
"24": 1.7132450748239665,
"25": 1.9464625152460222,
"26": 68.14622793558128
}
You may be better off on a different channel.
Some misconceptions here, maybe? When you “add devices via this device” you’re adding them to the network as a whole, not to that device. They will make their own connections, and those will probably change over time.
It will depend on the layout and construction of your home, but 10 routers out of 60 devices is not many. For comparison, I have about 70 devices, 30 of which are routers (though I have one or two very thick walls).
Edit: Have you read this?
The Cookbook in Community Guides has a big section on Zigbee.