Re: congestion in both the 2.4 GHz and 5 GHz bands
Do you mean using channels 1, 5, 9 and 13 rather than just 1, 6 and 11 in the 2.4GHz band?
Yes, I've heard that usually works well if everything in range can agree on it.
In the UK most 5GHz Wi-Fi when set to automatically choose a channel will try the lowest channels first (the 80MHz block formed from channels 36, 40, 44 and 48) because those are the only channels where DFS rules do not apply. These rules mean that with all the rest of the 5GHz channels you have to listen and check for absence of radar signals before and while using them. Even if you configure a UK Wi-Fi access point to use higher channels interference that makes the device "think" it hears radar signals can send it back down to channel 36.