I have everything connected to a google wifi, they choose 5 or 2.4 Ghz, I can't choose, no problems with 40 wifi devices connected when we are out of home, more if we are at home.
About the switches, z wave are more common, I have z wave switches, zigbee plugs and bulbs, wifi bulbs and a separate zigbee mesh for other devices not fully compatible with HA protocol. No interference here, everything works.
Sometimes Sengled has better price than Yeelight, just search in Amazon. They are good bulbs.
