Wi-Fi disconnecting when "under load"
I was eventually able to resolve it by following the suggestions from Bluetooth headphones disconnect a few seconds after connecting on macOS, namely:  


https://apple.stackexchange.com/a/344552 by Gummibando:


  Try resetting the Bluetooth module.
  Shift-Option click the BT menu bar item > Debug > Reset the Bluetooth 
    module.

https://apple.stackexchange.com/a/380827 by Kundan Burnwal:


  
  Delete /Library/Preferences/com.apple.Bluetooth.plist
  Clear PRAM according to Apple's instructions by rebooting and holding down Command +Option+P+R for roughly 20 seconds until either the second startup sound plays or Apple logo appears and disappears for a second time.
  



P.S. I think it is worth noting that the steps for resetting NVRAM and PRAM are the same, and did already try "the former", but that time it coincided with a minor macOS version update, which could have affected the reset procedure in some way.

Upd: unfortunately, pairing back and connecting a Bluetooth device (e.g., headphones) brings the issue back, so this whole workaround is just that — a workaround.