OK, so I've still been having problems with our Wifi. The big insight has been the turning on of Wifi AI and that at least our Windows machine can't handle the so called DFS channels in 5GHz.

The other thing is that I suspect there is a problem with longer range 5GHz where our Apple devices would suddenly have invalid SSIDs even though they work fine close up. And the signal would be very low.

The second realization though was that there is problem with DFS in that it allows only 250mW of power vs 1,000mW. So to test this I turned off WiFi AI and then did a manual channel allocation.

In looking at this, Unifi Wifi AI likes picking DFS channels but I'm not sure the power transmit is really taken into account nor client compatibility. How could it be, so I tried to do manual frequency reallocation and power allocation.

Here is what I learned:

1. 2.4 GHz Channel width and non-overlap. For 2.4GHz, you get a choice of 20MHz and 40MHz. Picking 20MHz is really the only practical way in that you get three channels you can use 1, 6 and 11. The main thing is to see where the interference is and avoid that channel. And if you have multiple APs, you have a self-interference problem, so you normally want your close UPs to be on different channels. The map kind of looks like a honeycomb where you want neighboring APs to be on different channels. The Unifi AI likes to pick overlapped channels, presumably to avoid interference between two.
2. 5GHZ Channel and overlap. 80MHz is the largest that's practical because you only get two channels. One on channel 36 and one on channel 149. Unifi as an aside uses a different notation for this, there is actually a 20MHz backoff channel and a 40MHz backoff channel, so they express that so the channel starting at Channel 149 which you enter in the UI comes out confusingly as 155 (149,+1)and 42 (36,+1)
3. Radio transmit power. Confusingly, Unifi has a default called Auto which is actually the same as High. The main thing is that you don't want everything high. If the APs are close together then you want as low as possible to prevent interferences. In a home, there are normally a small number of APs that need to be at high power because of dead spots or if you want your wifi to work outside. So turn those up and then use the rest as the lowest power you can stand to act as "fill in"
4. RSSI and Noise at your clients. What's all this then about whether this works. If you Alt-click on the wifi signal icon of your Apple machine, you get into the goo. The long and short is the confusing thing is that the lower RSSI the better. It's actually a negative number, so -36dBm is better than -90dBM. It stands for a relative signal strength indicator. And it gives you a sense of how much a client thinks it has in terms of signal. The less negative, the better. -72dBM is considered good. The other figure is how much noise there is. -90dBM is considered good. The difference by the way shows how clear the channel is. You want a difference of 20dBM. You have to go to each client to look at this sadly. but on MacOS, they give you the actually dynamic capacity, not just the reated one. It is tied to MCS class which is basically how the radio is operating. Generally, the higher the better.
5. Signal at the Unifi level. They are measuring what the APs actually see. Really good number for signal is -43dBM. They conveniently translate that into a percentage, but this shows you how the AP is feeling. You want them all at 1000% of course.