Optimal 5 ghz channel?

The Speed Test is:

• The number reported by the Orbi system (web interface or Orbi app), or
• Connecting to Ookla Speedtest on the iMac?

The 5G channel setting is far more complicated than one might imagine.  After spending a lot of time searching for a simple explanation, I am more confused than when I began.(When your eyes glaze over, just stop reading and say, "this guy is nuts")

To start with, WiFi "channels" have specific radio frequencies.  Wikipedia has a nice description: https://en.wikipedia.org/wiki/List_of_WLAN_channels#5_GHz_(802.11a/h/j/n/ac/ax) 

Here's what it says about channels 36-48 that can be selected in the Orbi app:

The green "Yes" indicates that these channels are available to use in the United States.  (The article lists regulations in most parts of the world.)

Notice that each separate channel does overlap with the channel next to it, but those channels are not listed in the pull down menu to be selected in the settings menu.  i.e. only 36, 40, 44, 48 can be chosen and none of them overlap with each other.

The issue is that a single channel is only 20Mhz wide. The 750 product's 5G radio is described as:

In order to create that (theoretical) blazing 1200Mbps speed, Radio 2 will have to be set to:

• A full 80Mhz wide radio channel
• Using 802.11ax encoding at 1024QAM
• Using 2x2 MIMO (two antennas at the same time)

When using only a single 20Mhz wide channel, the Orbi cannot possibly transmit that full speed, so the "bottom line" is that no what channel the user has selected, the Orbi is going to attempt to use ALL of the channels any time it can.

I think that the user channel setting dictates which single 20Mhz channel the WiFi "Beacon Frames" are sent over.  These frames are always sent at the lowest accepted speed so that even the least capable device on the network can receive them.

Beacon Frame Wiki 

So, (a) have not answered the question, and (b) have no advice to give. (sigh)