Lupine, I concur with Falcowe - your CPU seems to be running at max turbo boost and the fans blowing hard as hell is evidence of that. However, I too am very puzzled that your CPU monitor is not showing much evidence of cores being pushed to the limit.
I've sometimes used the (free) Intel Power Gadget to monitor CPU clock speed and thermal stress.
It is significant that you've found the same thing happening with this project running in your laptop instead of your iMac; because that appears to eliminate the possibility that your iMac has a system fault.
I'm currently running a VEPro-based project probably not greatly different from your project, in my 2017 iMac (4.2 GHz 7700K 4 core CPU), but I've never encountered the CPU mayhem on idle that you've described. Idle on my system is pretty relaxed. Most of my virtual instruments in VEPro are in VSL Synchron Players, with a couple of dozen Kontakts. But I don't have MIR Pro. Also, in all my Synchron Players the convolution reverb is switched off on every mixer channel - because large numbers of them can very easily demand far too much CPU.
One investigative procedure I'd suggest you try is to disable VEPro instances, one at a time, until you have none enabled. If then there's still CPU mayhem on idle, obviously we need to look elsewhere. If there's one VEPro instance that seems to be the culprit, disable each of its channels, one by one, to see if the problem can be localised in that way. Also, try completely disabling MIR Pro - indeed try that before anything else.
[Edit] I wonder if it might help to run your iMac's Activity Monitor, with CPU HIstory enabled (in the Window menu of the top menu bar). Although even on fastest refresh rate these CPU History graphs can't show any very fast intermittent spikes of activity (which I suspect may be what is happening in your project), they might nevertheless give you some additional insight.