Storage Full Error

+1 on the reboot.

In fact make that first reboot one into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

and you should reboot at least every other day. Doesn't take long and will help with performance.

unfortunately, your issue isn't software. it's your hardware.

when big sur is installed on the 1TB fusion drive, the macOS ends up moving to the very slow HDD portion instead of the VERY TINY SSD portion.

to improve performance, you should consider running an external SSD with big sur installed. to learn how to do that, please read this well written user tip from another member to these forums. How to Setup and Use an External SSD as your Startup Disk - User Tip

i'm thinking you should download and run the free version of EtreCheck so we can see if you have some software installed that is causing your issue. make sure you give "full disk access" to etrecheck. read how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

Just be certain 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 )

Running this way ( Full Disk Access ) will give a more complete and comprehensive overview of things.

i understand the hesitancy towards running your iMac on an external SSD full time. there is an alternative. you could have a professional replace the internal 28 GB SSD blade with a larger one. but it is labour intensive, and will not be cheap. the hardest part is removing the display without actually cracking it. if you would like, you can go to OWC's website and search for SSD upgrade parts. there you will find install videos for your specific iMac. then you will know what is involved for that operation and wether or not you want to go that route. best of luck.

you're welcome. please feel free to come back and let us know of your progress wether or not you need our help with your report. :)

Start by uninstalling your VPN clients then reboot. It has been 67 days since a boot.