@Swiftburn 

Ok from last to first.... 

Your channel lists are based on 20Mhz.  If you leave the AP on Auto, or pick a width greater than 20Mhz, it consumes the next adjacent channel (if available in your geographical location).  So for instance 20Mhz on 149, or 40Mhz on 149+153 or 80Mhz on 149+153+157+161.  Consequently 80Mhz is not really an option if your base channel is 161.  Further if you have an AP on 149 with anything other than a 20Mhz BW setting you need to make sure adjacent APs won't interfere or be interfered with.... ie don't set the AP in the next room to be on 153, or if you do, fix the unit on 149 to be 20Mhz so your frequency plan is stable and repeatable.

The earlier questions all require better data from you, for instance:

1. what are the signal levels actually?  If you are sitting beside an active AP, you should expect around -40dBm..... -85dBm or lower I would agree is pretty bad signal.  There's more than just signal level (RSSI), there's also RSRP, RSCP, Signal/Noise ratio, CIQ.... it's possible to have a blindingly strong signal that's being just as equally strongly interfered with.

2. what is your network topology.... ie what are APs connected to, what is that connected to etc.

3. are you using an Apple device for testing.... these have known issues in certain configurations.... so find a non-Apple device to do actual testing with initially

  @Swiftburn 

What are your nominal ping times?  How do they compare to wired ping times?

What is the typical gap between extended ping times ie the gaps (seconds, minutes..... )

Can you share your Netspot heatmap...I can't believe the APs suddenly are not delivering the same radio power

I'll be honest, those sporadically long ping times speak to packet loss...since your wired connection is fine, I don't think we can blame your ISP.  It's something in the local environment...maybe your neighbour's situation has changed?

Any chance you have some kind of packet storm going on in your switch?  Any anomalous bandwidth or packet count stats coming off the switch or APs?  Even a bad cable somewhere could cause all sorts of retries generating a lot of extra traffic/cpu usage.

I don't know if it's an option, but it would be interesting to test JUST a single AP connected directly to your router (with the switch and other APs temporarily disconnected).  Test performance, and then start to re-introduce the rest of your network.

  @Swiftburn 

I can respect the 24yrs...I am the same 

It is possible that a bad batch of capacitors coupling the radio amp to the internal antenna have cause a significant reduction in radio power...which would cause this effect you are seeing, or similar.  1yr doesn't fit the typical bathtub failure curve though.  Have you had any big power surges or lightning strikes nearby?  That too could blow the output stage of your radios...and would be a better explanation for them all to fail.

I would still like to see actual measured power as shown by Netspot...a screen grab added as an image to your post works well.