Paul,
I've set this up with an ASI120MC-S in a security camera dome (awaiting delivery of a better acrylic dome), but the last 2 nights, it has been covered in dew. It's doing a little better tonight, but the night is young.
http://allskycam.sqlsolutions.com.au/
The RPi4 that I'm using is currently inside the dome, and I'm running a USB cable only out to the camera.
I know the RPi could potentially be mounted in with the camera in a box, which would supply some heat. In your configuration, does that provide sufficient heat to ward off the dew, or do you attempt to do it by other means?
For what it's worth, I've put the PoE HAT to power the RPi, which is a good 1-cable way of powering the unit, proving ethernet in plenty of juice for the camera. A bit of heat pops out of it too!