As has been said above, your temperatures are not excessive. Furthermore, it sounds like your machine is locking up - becoming too busy to respond to inputs - rather than crashing. In my experience, fans getting noisy when Rhino running is not uncommon.
I would start thinking about other reasons for this behaviour.
First up, consider the file. Do you have a colleague or contact with Rhino who could edit it to see whether they get similar behaviour? If they do, it’s the file, otherwise it is your system.
Assuming it isn’t the file, check your Windows event logs for errors coinciding with your lock ups. You may find a clue.
Check your firmware and drivers are up to date.
Check your Windows Update history. Did the problem start coincide with a particular update installing? Google it for similar problems. Are there updates failing? Time for a Windows repair.
Open your PC and methodically remove and replace cards, connectors and memory sticks to ensure everything is properly seated and you have good electric connections.
Download the memtest utility and check your memory is all good.
Do only if you actually know what you are doing. If you don’t have someone with expertise do it.
While you do this step also use compressed air to blow out any heatsink fins, fans and such. Blow out every nook, gap and grill you can find, even on your case and the radiator of your AIO. Thermals aren’t the problem, but once you get in your machine it is a great time to actually do some good cleaning. Depending on your environment you might already see some dust accumulation inside the case on parts and surfaces (:
On NVIDIA’s advice, I reinstalled the studio driver as a custom install with the clean install option, that basically started over with the original settings. After that, it still has the sudden temperature jumps with fans full out and might stall the model rotation in the viewport, but it no longer freezes the system, and recovers its rotation after a few seconds. So I can live with that. It is an aging system, purchased in 2019.
I truly appreciate every idea and tried every one that I could.
It is apparent on opening the cabinet that it was not designed for air flow. The GPU is indeed a fraction of an inch below the power supply toward the rear. The cabinet fan is located in the front behind a number of airflow barriers and does not appear to activate when the GPU fans activate, so the GPU heat must just circulate inside the cabinet. Incoming air for the GPU is provided through a circle of perforations on one side of the shell with the GPU fans facing down at a right angle to the little openings and to the exit through the non-running cabinet fan blades. Not exactly an optimum design for air flow.
Many thanks again to you all for your incredible insights!