I upgraded the OS to Big Sur.

Thank you.

Then, really really ( no typo there ) make a Time Machine Backup right away.

Even Better , make two independent TM Backups each to separate External Drives

Scorched Earth Method 

This method will WIPE ALL Data and there is No Recovery - Period.

>> Only works on Intel Based Apple Computer <<

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example ONLY : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

9 - Use Migrate Assist to bring back all your Personal Data

Transfer to a new Mac with Migration Assistant - Apple Support (CA)

What follows in Not intended to be a flippant reply.

User choice to work through the existing issues in macOS 11 Big Sur or Wipe the Drive and start back with macOs 10.14 Mojave.

Putting aside, for the moment, the seemingly daunting Technical steps to downgrade back to Mojave

Doing back would put in jeopardy your ability to use the Old Version of M.S. Office.

From what I have only read about this, Microsoft only offers Licensing for the Current versions of Office

If the computer has so many issues aside from MS Office for Mac, which you seemed to have resolved by using LibreOffice .

I would be inclined to stay with Bug Sur

That said - a neet application for seeing what and where some issues may be occurring >>

Download the Application Etrecheck directly from the Developer.

This is a Diagnostic Tool that makes no changes to the computer.

It makes a coherent and readable inventory of both the Hardware and Software used on the computer 

The application is free or paid from added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues

A late 2013 iMac doesn't support Big Sur.

The first thing you would need to do is create a Time Machine backup.

How about a Screen Shot of " About this Mac " without the Serial Number appearing

DreamtimeAquarium wrote:

Thanks again, but Etrecheck is not on the Appstore and some sites say it's malware.

Don't know where you read that but it's total nonsense. EtreCheck is the most recommended app for diagnosing problems on ASC.

As my Learned Colleague has attempted to enlighten you on the usage of Etrecheck Application a couple of things come to mind.

I am aware of an Internet Posting outside of these Forums making that specific claim about Etrecheck Application.

The fallacy in that logic is, 1 is not ALL in regards Etrecheck.

More often than not the same type of posting on the Internet will say macOS Requires Antivirus Software to be safe

Then we move onto a specific Disk Cleaner which shall remain un-named

All said and done, good luck with this computer issue

Have taken up enough of your time

As my Learned Colleague has attempted to enlighten you on the usage of Etrecheck Application a couple of things come to mind.

I am aware of an Internet Posting outside of these Forums making that specific claim about Etrecheck Application.

The fallacy in that logic is, 1 is not ALL in regards Etrecheck.

More often than not the same type of posting on the Internet will say macOS Requires Antivirus Software to be safe - fallacy

Then we move onto a specific Disk Cleaner which shall remain un-named - also a fallacy

1 - Disk Cleaner 

This will include CleanMyMac , aka “ BrickMyMac “

MacKeeper,

Dr Cleaner Pro Plus

Open Cleaner one Pro

Macmechanic

2  - Third Party Security Software 

This will include BitDefender , 

This will included Norton Antivirus 

 Sophos Av Software

Intego AntiVirus

 McAfee, 

Avast AntiVirus

Ad Guard,

 Webroot ,

 ESET ,

 Avira ,

AVG AntiVirus  

 avira antivirus 

Trustee  

AntiVirus for Mac

F-Secure

Securemac

Cylance

Kaspersky Internet Security

Total Virus Software 

Read some of the posting and arrive at your own conclusions.

The The Built in Security  is all that is required.

All said and done, good luck with this computer issue

Have taken up enough of your time

EDITED

FWIW, a MacBook Pro is not an iMac.

Just to give you an idea, EtreCheck is referred to at least 39,800 times on here.

You might want to read this as well: Using EtreCheck - Apple Community