My plan (I have bought parts but haven't received them yet) is to make a Pi Zero HQ webcam (NOT running octopi), that then connects to a Pi 3B+ (which IS running Octopi) over USB as a USB webcam. This is because I want to be able to use the (somewhat expensive) HQ camera for other purposes, and didn't want to keep plugging/unplugging the camera ribbon cable from my Octopi 3B+. Is anyone else here doing this? (Is my description of what I plan to do clear?) Is using a webcam in Octopi more taxing on the Pi than using the native ribbon connected camera?
This should work ok. I have used a Pi Zero with a Raspberry Cam (though not the HQ one) as an external camera for OctoPrint running on another pi in the past and did not have problems. The advantage is that even if the zero is having its wifi/cpu issues, it will not affect OctoPrint because that runs on a different pi. At the most, you will not have a camera view and/or your timelapse fails when this happens.
This doesn't work quite how you think it will, but the overall plan itself is sound.
There is such software as MotionEye(OS) that exists solely to stream webcams etc. You can install this on the Pi zero, or install something like mjpg streamer (same as OctoPi has) that serves the webcam. Then all you have to do is change the URL in the OctoPrint settings. Webcam streaming is not done by OP itself, but it just embeds the stream from a different provider.
There's even an app (definitely Android, unsure about iOS) called IP webcam, that you can use to do this if you have an old phone somewhere. Most even 5 years old have more than decent cameras, and may be gathering dust.
You can actually masquerade/use a Raspberry Pi Zero as a USB webcam:
Interesting - wasn't aware that was an option. TIL. Thanks!