For me (and at least one other commenter in this thread), the issue occurs when plugged into an external monitor. The linked thread seems to indicate the same. Any thoughts on that? I'll check the relevant setting.

I think graphics switching only works when running on battery power. Wonder if there's a way of make it still work when plugged in...

Ahh - probably true. It was annoying me playing world of warcraft and I turned down the settings to shut it up - I forgot that.

Not an option if you are using it for your "work" workflow.

There must be a kill switch somewhere; I'll have a look.

right - I've had a look and I found https://github.com/codykrieger/gfxCardStatus.

I cloned it and then compiled it with xcode. I found one problem which is a badly encoded file, which I fixed using the file inspector to set the encoding and then saved it. After that it built and runs. On my 16' 2020 MBP it allows me to turn off my 5300M. When I open WOW it refuses to allow the card to be turned off, its web site says that's what it will do with an external monitor too.