I installed 3 R600 unleashed access points in a 3500 sq ft 2 story home.
2 units downstairs where there is more square footage and on opposite sides of home and 1 unit centered on second floor ceiling.
I am curious as to best practices as the units that were replaced were older UniFi "G/N" units and were not nearly as powerful.
I noticed that with the R600's all set to auto (default configuration) connected clients all over the home seem to hang on to that one access point upstairs even though there is one much closer and with zero load. Peak wifi load would be around 30 devices at any one time (connected rokus, apple tv, iPads, iPhones, smart TVs, Nest thermostats, Sonos player, when company is over, etc.
My initial thought was to lower the power of each radio to low or perhaps adjust the band steering settings to a much lower percentage (5%?)
The client devices are perhaps getting enough strong signal to stay connected to the AP upstairs.. if I were in your position I would perhaps reduce the Tx power on second floor AP to half or lower so that clients that are closer to it only connect and rest connect to APs on 1st floor. Bandsteering is used for distributing clients over 2.4G and 5G radio and you do not need to enable as per the above context.