So far I have had the best luck with one SSID (not counting a guest network), Band Steering enabled but not Fast Roaming. Behavior is that you stay on 5GHz unless the device doesn't support it, or are out of range. The client device will manage switching between APs. Some client devices like to hold onto poor connections, but if you use Fast Roaming instead, it will forcibly disconnect you which will create a brief interruption. If you let the client device manage the connection, there doesn't seem to be an interruption. If the AP density is too high with Fast Roaming on, the client device will constantly be getting dejected from various APs which will create issues especially on WiFi calling.

Another thing to look at is radio transmit power under advanced wireless. The default setting is Full Power, but sometimes it helps to turn it down so that your client device is not always trying to connect (or maintain a connection) to an AP that could be out of it's own range.

As you can see, there is a lot to go wrong, and that's why I have found physical location to be more important than any particular setting.