@Hebla 

Guest network is not the feature of mesh system. Many modern standalone WiFi routers offer Guest network. It is used exactly how you think it would: for guests to connect to. They can have access to Internet, but not to your home network.

Deco provides that feature, too.

It works better if you keep Deco mesh running in Router mode (which is its default mode) and you set password for Guest network (when you enable it, by default it is without password).

Current X60 firmware does not support bandwidth throttling, it will come in the future with firmware 1.6. With that firmware, you'll have "schedule and bandwidth limit to guest network under Deco App > More > Wi-Fi > Guest Network. Note: This feature is ONLY supported when the Deco with the firmware 1.6.0 installed is set as the Main Deco."

There is no ETA for that firmware for X60. You may have to manage till then. An option would be to configure Guest network to only run on 2.4GHz (disable 5GHz on Guest network). That might not throttle devices on guest network as much as you want to, but would somewhat limit single bandwidth hungry device.

I am also assuming you have unlimited Internet, having metered would be really unwise in your case.