update to AppleConnect 3.5

Did you get your Mac from an Apple employee second hand? Apple Connect is an app to let Apple employees connect like Zoom. Look in your System/Users & Groups/Login Items so see if it's listed there. If it is uninstall and reboot. Then try to install Big Sur.

You can also look for it in the Applications folder. If you find it move to the Trash bin and empty.

You don't, however if you would explain the issue you are having while trying to upgrade your 2014 iMac to Big Sur that would be helpful. Then we can begin troubleshooting the real issue.

Please post a fresh Etrecheck report and we will look for more issues and also check to see if all of the Symantec garbage is gone.

Good job on getting rid of Symantec, that appears gone. Your last report indicates it is possible you have some adware installed. To get of any download and run Malwarewarebytes.com, get the free version. Then again restart in Safe Mode and then fully shut down and restart normally, however this time hold down the power button for 15 seconds before releasing it.

It sounds like the next thing to try is resetting the NVRAM and then restart again while holding down the power button for 15 seconds. If it still won't start you may have a hardware issue. How to reset NVRAM on your Mac

Okay, lets start with some basics. Please do the following:

Please navigate to the Mac App Store or 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

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for Symantec and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally and begin to install Big Sur.

Additionally, have a look at this thread re. AppleConnect:

https://discussions.apple.com/thread/250722681

And, as an aside, you appear to have a ton of 32 bit apps installed - none of them work in Catalina or Big Sur. I'd try to uninstall them as they are just extra/needless baggage.