I realised after pressing submit that toolchain was the wrong word.
Anyway, thanks for your help. All is well now.
I think I know what I'm doing, but that's clearly an illusion...

As can be seen in the original post, the toolchain used uses Python2. pip3 is not going to get pySerial for python2.
The way forward, ideally, is for the toolchain to be upgraded to Python3.

I have just installed Ubuntu 20.04, and learned to my dismay that Python2 is only marginally supported. pySerial is not included, nor do I see any way to install it. It's not in the repo, and pip2 is not available either. The boards manager URL I use is https://dl.espressif.com/dl/package_esp32_inde...