Murphy's Law!!!
I have previously extensively reported on how I had the same problem of lisign wi-fi, tempo solved by using an ethernet cable to a wi-fi extender.
Following a contact from a very charming Apple support person a dn gping through diagnostics with him, I unplugged the ethernet cable and everything was working fine - and high speed internet has been maintained for several days now.
So there are differences between when it was not working and now whne it is:
First, I have two identical HP27 monitors attached that were originally connected with 1) an HDMI-HDMI cable to the HDMI port, and the second with an HDMI cable plugged into a Thunderbolt port via an Apple USB-C dongle (the one that goes from Thunderbolt to HDMI, USB-2 and USB-C ports). I replaced both of these with new 1m Silkland USB C to HDMI 4k cables purchased from Amazon (described as [2021 Updated, Optimal Chip] USB C to HDMI Cable (Thunderbolt 3 compatible) for MacBook Pro/Air), both now plugged into Thuderbolt ports.
Second, I did the Apple Security update so I am running Venture 13.3.1 (a)
Now I do not know which, if either, of these changes has made the difference, but I am not suffering from the wi-fi drop-outs any more.