Migrating from a "PROBLEM" iMac to a brand new iMac ... ?

First you need to get the old system stable before you migrate any data from it. However before can do that we need to know what is installed on it. You have listed Clean My Mac and also Norton, which are both problematic at best. So we can get an idea of what you are dealing with please do the following:

Please run an EtreCheck Report. Please click http://www.etrecheck.com/ where you will find the app. Once you downloaded the app and created the report please post it as your reply to this post. In order to post it, please use the following link for more detailed instructions on how to correctly post the report:

https://discussions.apple.com/docs/DOC-250000211

We can then look for obvious issues that may be causing your problem.

The following MUST be uninstalled using the developers uninstall instructions:

CleanMyMac4

Norton products

To be honest you created quite a mess. Perhaps a little education will help you in the future.

Mac OS is EXTREMELY secure and does not require or benefit by using any third party antivirus, cleaning or ANY third party "maintenance" apps. Almost all will create the exact reverse affect, making Mac OS unstable, slow and appear buggy . If simply kept up-to-date and otherwise left alone Mac OS will run stably and securely for most users for years!

There are zero viruses for Mac OS, however there are a small amount of other types of malware out there, however these are very easily avoided by using some common sense such as never using a Torrent to download software you did not pay for, never download from a site you do not trust, don't fall for scams that "advise" your system has been compromised, these can come as phone calls, emails, popups, mailers, etc. You have to download Mac OS malware, using common sense keeps your eyes open.

Good luck!

You can check to see if you've removed all of the files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For example for CleanMyMac you'd do two searches:

1 - Name contains cleanmymac

2 - Name contains macpaw

Any files that are found can be dragged from the search results window to the Desktop for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.

For the PACE software search for Name contains pace.

For Norton search for Name contains norton.

Boot into Safe Mode, Use safe mode to isolate issues with your Mac, and try to delete that norton file. Safe Mode disables all 3rd party software. It's boot time will be longer as it cleans out cache and swap files and runs some diagnostics.

PACE is anti-piracy software which is unneeded and just runs in the background and uses up system resources. You can search for PACE uninstall directions but only use manual directions. Don't download any removal apps.

Please follow the directions in:

https://cleanmymac.com/how-to-uninstall-cleanmymac-3

AND

https://support.norton.com/sp/en/us/home/current/solutions/v64924250_EndUserProfile_en_us

You read the second link I provided, that explains how.

Uninstall the FaceBook Cleaner and CleanMyMac, locate the developers instructions to uninstall both and uninstall them. I did not see any antivirus apps installed, however if you have any installed uninstall by the same method, use the developers instructions. After both are gone, then restart in Safe Mode (Use safe mode to isolate issues with your Mac - Apple Support ) and then restart normally and post a fresh Etrecheck report.