Thanks, everyone, who contributed to this thread. I've been having the same issues described (iMac Pro 2017, macOS Monterey 12.2.1, no external monitor) for the last couple of months and — like others — had tried all the usual diagnostics and resets without luck. I couldn't figure out what had changed. My machine has been super stable for the last 4 years.
After reading the last few pages of this thread, I identified something. Over the Christmas break, I spent time identifying people in my large iCloud Photos library. I had never really used the People feature, but went through manually adding names, then confirming the suggestions. Now I have over 100 people identified. While that didn't obviously start the kernel panics when idle. I'm now wondering if it's whenever new photos get added to iCloud. About 1–3 times a week, I get to my desk in the morning and my iMac Pro has reset with the same watchdog-induced kernel panic. I'm trying to match this with my behavior, but — as happened last night — my current theory is that it's triggered when I go out the evening before — while my iMac Pro is idling — see friends, take a photo of them/us with my iPhone. That photo goes to iCloud, then to my iMac Pro and that triggers whatever the bug is with the background analysis threads.