iMac Restarting in Sleep Mode

What I see in your report is:

1. "No Time Machine backup - Time Machine backup not found." I also do not see any evidence that you back up the computer such as any back up apps. If this is correct (that you do not back up the computer) that needs to be fixed as your #1 priority. At the very least you should be using Time Machine ( Back up your Mac with Time Machine ) which requires and External Hard Disk to backup to. IF you do not have a spare External Hard Disk that you can dedicate to Time Machine then many experienced users here like the OWC Mercury Elite Pro.
2. "Unsigned files - There are unsigned software files installed that could be adware and should be reviewed." You might have some adware installed, I would recommend that you download the free version of Malwarebytes (malwarebytes.com) and run it to see if it finds any adware.
3. "Kernel panics - This system has experienced kernel panics. This could be a sign of hardware failure." After you backup please post your last KP report and we can look for any evidence it was caused by software. KP's can be tough to figure out as they can be caused by hardware or software. Common hardware causes are failing HD's or RAM that may have failed. As you have not upgraded the RAM on your machine it's a long shot but it is possible the SSD is failing.

Looking through the report I do not see any other obvious issues.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

Disable Sleep, it’s unnecessary on later Macs, all pain, no gain. To disable, check “Prevent…” in Energy Preferences and uncheck everything else. You can leave the display off time as is, unrelated to Sleep. Shut for long absences i.e. > 2 hours.

I suspect the internal HD may be failing. Please get DriveDx and test your drives. If I am correct and the HD is failing that would explain the KPs and would be something you need to address.

If I am incorrect then you should create a new user account call it TEST ACCOUNT and run the machine using the TEST ACCOUNT for about a week. If the KPs vanish while in the TEST ACCOUNT that meaning there is an issue with your normal user account and that is where you need to focus. If the KP's' continue while in TEST ACCOUNT then it's a global issue (such as the failing HD mentioned above) and that is where you need to focus your efforts.

Can you please post a snapshot of your Energy Saver Preferences?

I stopped sleeping my iMac a couple years ago, the modern versions of Mac OS are just too buggy with sleep/wake issues.

Great! You’re welcome!