Excellent! Glad that finally resolved it.
Because the last steps I gave can be intimidating to most folks, I wanted to exhaust the less invasive processes first. Deleting things in those folders can cause issues, unless specific steps are followed. As always, it is best to have some form of current backup of your data before proceeding, which you had mentioned Time Machine at pone point.
1) I am running Big Sur on one of my Macs, and don't have any real issues. However, it is not my main Mac. It would depend on what kinds of things you do, what versions of any older software you have, etc.
2) Personally, I don't recommend any antivirus or third-party automatic cleaning utilities. Most of them cause more issues than they are worth. Sophos is used more by business/education to protect their equipment. Symantec/McAfee/BitDefender/Kapersky/etc. do work, but can be a bear to uninstall or disable real-time scanning.
That said, if you really want some:
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Malwarebytes for Mac is great for malware removal - you can just use the free version.
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ClamXAV is the best and least intrusive antivirus I have used - and can be run in manual mode.
I always recommend that Malwarebytes and ClamXAV be run in manual mode, and have any real-time or background scanning disabled, to prevent overall slowness.
Hope that helps.
C
