Software update tab is stuck on "Checking for Updates"

1 - Suggest restarting in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

4 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

5 - Suggest downloading the Application Etrecheck directly from a well Respected ASC Contributor. And Safe to use.

The application is free or paid from added features. 

Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).

It will take a Snap Shot -  both the hardware and software.

 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.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS in-order to replace any modified, altered, removed or corrupted elements of the OS this software has inflected on this computer 

Although some or all of these Applications maybe available on the Apple Apps Store, it does not mean Apple has verified what exactly the Application can do, does do or is doing to the Computer. That responsibility is left to the User to perform their due diligence on the Application before purchasing and installing it.

Any of the below should be removed as per Developers Instructions 

This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.

This will include CleanMyMac , This will include BitDefender , 

This will included Norton Antivirus , Sophos Av Software

Intego AntiVirus, McAfee, MacKeeper, Avast AntiVirus

Ad Guard, Webroot

The The Built in Security  is all that is required.

Hello Fargo622.

Thank you for using Apple Support Communities. Try updating your iMac while in safe mode. It sounds like you're already familiar on how to start up in safe mode, just in case the steps are here: How to use safe mode on your Mac - Apple Support

If you're using Apple silicon:

1.) Shut down your Mac

2.) Turn on your Mac and continue to press and hold the power button until you see the startup options window.

3.) Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”4.) Log in to your Mac. You might be asked to log in again.

If you're using an Intel processor:

1.) Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.

2.) Release the key when you see the login window, then log in to your Mac. 

3.) You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

If an error populates while updating your macOS, this resource can also help: If an error occurred while updating or installing macOS - Apple Support

If you're still unable to update in safe mode, try reinstalling your macOS next to see if that also helps: How to reinstall macOS - Apple Support

Start up from macOS Recovery:

Apple silicon:

Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labeled Options, then click Continue.

Intel Processor:

Turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.

Once in macOS Recovery, you can select "Reinstall macOS". Feel free to keep us updated on your results. Cheers.

This has been an ongoing but intermittent problem for weeks.

Is there anything I can do about this?

Try another time or wait for Apple to fix it.