Thanks for the input. I'll only transfer my User Account over to the new computer once it arrives.
Regarding the Aura chip, this was a refurbished unit I bought directly from Apple about 2018 so I don't know how/why it got installed. I transferred the data to this mid-2015 model from my old 2011 computer, so that might explain some of the really old files and extensions.
BTW, I only just installed Catalina not Monterey on this computer when the Apple Mail problem started.
I'll delete Cleanmymac and do a re-install of the Catalina OS as you've recommended. If that doesn't work, I can always re-install the clone I made before the upgrade to Catalina, never had any issues with Mojave.
Are there any third party programs that you would recommend for Optimization, Cleaning, etc.? Or just avoid them?
Again, thanks for the insights.