iMac freezing and crashing

Good followup info. Thank you.

I noticed in a forum that someone had mentioned excess CPU usage from the Photos app and solved it with AppTamer so I tried that.

I saw that in your first EC report but not the second, so did not mention it. You don't need a third party solution to fix that "photoanalysisd" problem--and it is a known problem. As crazy as it sounds, opening Photos will stop it. See this interesting outside article:

https://apple.stackexchange.com/questions/310594/what-is-photoanalysisd-and-why-is-it-using-77-of-my-cpu

I think you can now safely uninstall AppTamer after reading that article. I suspect AT falls under the "cleaning apps" pall.

Could using a second monitor be an issue? It's a Samsung TV. My previous iMac had a problem with its GPU overheating I think

It could. I know I've had a couple of MBPs show higher temps when I used an external monitor. The bigger the external screen, the more work the computer has to do. However, if you are casting to the TV or an set-top video box like an Apple TV, I don't think the demands on the computer are as high.

My 90 day Apple coverage is up, how to I speak to them?

Even Apple refurbs have a one-year warranty. You can confirm what coverage remains with this Apple support page:

Check Your Service and Support Coverage - Apple Support

I just ran Etrecheck 5.7.1, the highest version of the free version, on my iMac 5K from 2017 and it shows a non-zero value for i/o speed. It could be reporting error or a simple difference in whose Apple's SSD vendor is. If things get better otherwise, I'd not sweat it.

I still see CMM, or a component thereof.:

Launch Daemons:

[Loaded] com.macpaw.CleanMyMac4.Agent.plist (? 0 - installed )

We're chipping away at this.!

OK, I've asked this question 3 times now and got nothing, no one has even read my Etrecheck report. Am I invisible??????????

Not invisible but it could be attitude. This isn't social media, nor are you talking to Apple. You dumped on a long-serving and very experienced senior contributor who has read thousands of EtreCheck reports. Remember, no one here can see or touch your computer so we have to work only from reports like EtreCheck, user information, and our experience.

When you read as many of those as we have, you understand that all cleaning apps will slow and/or destabilize a modern Mac by conflicting with its with its sophisticated auto-maintenance routines programmed into the OS.

So I will put that behind us and work with the current report. Cleaning apps, especially CMM, have to be removed to prevent their masking other issues. I am glad so see CMM gone now.

This, regarding your SSD, concerns me a bit:

Performance:

System Load: 1.50 (1 min ago) 1.75 (5 min ago) 1.51 (15 min ago)

Nominal I/O speed: 0.00 MB/s

File system: 17.72 seconds

Write speed: 2830 MB/s

Read speed: 2165 MB/s

I collect EtreCheck drive reports and have never seen a nominal I/O speed reported as zero. You may want Apple to evaluate. There is a newer free version of Etrecheck available (5.7.1) that you may wish to run to see if the zero speed score is a reporting bug.

You have some apps installed I do not recognize but EtreCheck is not flagging them. What I see in your description is that the apps you were using at the time of shutdown are high-demand and, if the cooling vents on your iMac are impeded, the processor will slow to deal with the heat and, should that not help, the computer does a safety shutdown. So that is a possibility. A temp monitor test I ran on a 2017 iMac 27 5K shows that a one-hour Zoom meeting can raise CPU temps as high as those posted by high-demand, graphics-intense sims and games.

Chrome is a resource hog that can contribute to higher temps. I would not leave it open in the background.

Cooling-related things to check:

• Stuff stacked between the work surface and bottom of the screen blocks incoming cool air. \Keep that ~3-inch clearance at all times.
• Having the screen tipped fully forward restricts the flow of exhaust air, increasing temps.

I would start by insuring your computer can freely inhale and exhale. Then look at some of the older apps you have installed and see if they are really needed. I saw at least one that was three years older than your computer. Some older apps not updated have been reported to create stability issues.

This is an odd issue and may not have a "one-post" solution. There may be some back-and-forth to reach a solution or a recommendation to have Apple evaluate.