Flaky WiFi Operations with BR500 and WAC510s

Back in September 2019 I was having WiFi problems in the house (see quoted extract below).  I am having the same type problems again, even though I have replace the old Nighthawk Wireless Router with a BR500.  Periodically the Wifi starts dropping clients, failing to hold connections, and generally not operating correctly, all the while having the wired connections to the outside world working just fine.  It did it about 2 weeks ago and then the last couple of days.  Am really getting fed up with this Netgear stuff.  Here is the original post from September:

"WAC510 Firmware Upgrade 8.2.1.16 has been wreaking havoc with my home network.  I have four WAC510 devices, all of which were recently updated to firmware release 8.2.1.16.  They are connected to a Nighthawk Router that has its wireless turned off [SINCE REPLACED BY BR500].  Symptoms since the update include the following:

1. IOS devices showed the available SSIDs OK for connecting, but when chosen showed "no internet connection".  This problem is erratic.  The available connectivity drops easily, although in the last 12 hours the "no internet connectivity" problem has not recurred.

2.  Sonos devices have struggled to connect, although they seem to be recovering.

3.  Denos HEOS devices are erratic in their connectivity.

4.  Belkin WEMO devices cannot connect at all.  This is more complicated than appears, because even before the upgrade the WEMO devices were not integrating well with other non-WEMO devices via the Samsung Smartthings hub.

5.  Some, but not all, the Google Home devices cannot connect.

6.  Nest thermostats look like they are operating OK at the thermostat interface, but they show offline on the Nest IOS app.

7.  Nest cameras have a harder time connecting to the network even when near the WAC510."

All WEMO internet things are offline, the IOS devices have no internet connectivity, etc.  I am writing this post on my desktop PC which has a wired connection to my home network.

Any ideas?  Have firmware updates broken my system.  I have WAC510s at 8.2.5.1 and the BR500 at 5.8.0.1.  Two weeks ago the problem seemed to fix itself, which is never good (in my view) as it can just as easily break itself again.  I restarted the router and a couple of the APs just in case, but to no effect.