SLR as a Webcam
As I built out my home studio to record videos for my distributed classes, I was lucky enough to be able to find an in-stock HDMI capture card, but those are harder and harder to find. As it turns out, you may be able to avoid the need for that with a mix of apps.
This article on PetaPixel points to Camera Live (Maybe Canon only), Camtwist.
The key step in getting this to work with Zoom is
sudo codesign --remove-signature /Applications/zoom.us.app/
This is an odd step, explained as "first, you remove the “signature authorization” required by Zoom to recognize the camera as a webcam," but I don't think that's what that command does. (Entertainingly, while codesign is one of the relatively few bits of MacOS that comes with a fleshed-out manual page, the manual page doesn't document a remove-signature option.)
I'd thought that what this would do would be to allow Camera Live to drop a plugin into Zoom, and not have Zoom's signature break. iGlasses has worked for me for other conferencing apps, but not Zoom. After removing the signature and loading up iGlasses, it works in Zoom, but does not seem to drop anything into /Applications/Zoom.us.app or ~/Library/. However, that codesign command now breaks something in Zoom's ability to access the microphone, which I think is likely a self-integrity check. I've had to downgrade to an earlier version of Zoom.
Incidentally, there are HDMI capture cards on Amazon like this or this. Those are 1080p, not 4k, but generally, 1080p is plenty of resolution. In my own experiments, OBS starts to spin the fans if I record at 2550x1440 for a minute on my reasonably recent imac, and so I've been capping my video resolution and delivering the best quality sound - I suspect most people are watching in pretty small video windows anyway.