Just to wrap this one up, it seems my problem with a combination of Zoom and a USB hub was down to my Dericam webcam. I’ve replaced that with a Logitech C920 Pro, which has worked OK.
That’s a shame, because the Dericam has quite good image quality (though the microphone is poor) and it’s inexpensive (though I had to pay twice the current Amazon price of GBP 20).
To recap, the issue was video freezing in Zoom when the Dericam was connected to my 2015 MBP via a powered USB hub. Zoom was fine without the hub, and other video apps were fine with the hub.
It seems you get what you pay for. Who knew?