External USB webcam video/audio out of sync

I'm struggling on my High Sierra 2017 retina iMac 27 to get my new Sony ZV-1 camcorder to deliver synced audio and video via USB. To be fair, even the built-in FaceTime cam and mic are slightly out of sync but selecting the Sony cam and its mic as the input sources opens up a big can of worms as the audio comes in more than 1 second ahead of the video.

I've tried selecting different audio inputs (external USB lapel mic and the built-in iMac mic) but they don't improve things. The ZV-1 with its latest 2.01 firmware is definitely supposed to work directly via USB as a web cam without any additional software or hardware -- and indeed, I can force it to produce synced audio and video by selecting one of the alternative sound inputs and immediately switching back to the ZV-1 input. But I'm creating a lot of short 'selfie' talking head movies in Quicktime Player and when I complete one and start another, the video is lagging again. I would love to know why I can force it to work once though.

In my troubleshooting steps to try to normalise the USB stream, I tried the following (none of which helped):

1. Reboot computer

2. Reboot ZV-1

3. Clean my HD with both Disk First Aid and CleanMyMac

4. Logging in to my computer with a virgin test account

5. Trying two different USB cables

6. Trying two different USB ports

7. Switching to two alternative audio sources (internal iMac mic and external USB mic)

Would it be worth zapping P-RAM and/or resetting SMC?

Alternatively, am I overloading the USB port by feeding it 1920 x 1080 video at 30fps? It looks great, plays smoothly and only lacks one thing...

Grateful for any help. I can't find any other similar complaints about this camera so I assume the problem is in my setup - which is very basic as I don't have any external MIDI devices.