I have a 3x deco M5 mesh network setup right now and it works great in our 2k sqft two-story home. 

I'm moving my home office into a 400sqft cottage we have set back on our property about 200ft from the main house. I've run cat7 underground for hardwired desktop usage (we have gigabit fiber via google). This cat7 line is connected to port-2 on the google fiber router, port-1 connects the main deco M5 in the house. On the cottage side, the cat7 line connectsw to a 10-port ubuquiti edge switch.

So my goal is to also have wifi available in the cottage for laptops, mobile devices, smart security devices (i.e. cameras, moisture sensor, etc). I don't necessarily need the network to be strong in the backyard between buildings, so long as it's strong once I've reached the cottage.

Is it possible to simply connect another M5 to the ubiquiti switch? Or, given the abundance of connected devices out there, would I be better off setting up a new AP with a new SSID on a different WLAN channel?

The main house doesn't have any smart wireless security devices yet but it will in the near future. I'm considering setting up a new wireless AP for those devices on a hidden SSID.

I don't want to overcomplicate my hardware, but I also want optimal and reliable performance.