Massive cache attributed to CloudKit

My suggestion: get rid of CleanMy Mac asap per their uninstall instructions. It can, and does, disrupt operations.

After that, do Apple's suggestions again.

If all else fails, reinstall the OS. Takes maybe 40 minutes and doesn't touch apps, data or settings. Of course a setting may be the culprit so a fresh start may be in order. Or you could just not migrate settings and start that afresh.

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

And, If using Carbon Copy Cloner would suggest tweaking the SafetyNet Configuration in CCC as there might be addition Snap Shots lurking on the Internal Drive associated with CCC

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

Very Much agree with @ku4hx regarding CleanMyMac which is akin to Malware.

Yes, malware to be polite but would use stronger words if allowed on these Forums.

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster and comprising the Operating System

Any of the below should be removed as per Developers Instructions 

Do not Drag and Drop to Trash as this will make a bad situation even worse

This will include CleanMyMac 

Do suggest reading some of the first 600 posting specific to this Application and arrive at your own conclusions.

There has been sufficient advise offered, from more than one Contributor, for the User ( you ) to make an informed and educated choice what the next course of action is required.

The suggestions have been put forth on a volunteer basis, in good faith and in the best interests of the computer.

Would have to say, that would a guess at best, unless the User Allowed them Direct Access to this computer to Analyze the actually Internet Connection speed.

Even we, who are also working Remotely could not perform such an analysis of Internet Speed on that computer.

Did they know CMM was installed when they were connected ?

EDITED added information

GusDev wrote:

You write like a machine 😅

I, the User, can safely say my internet speed has been subpar in multiple locations where I had to back data up and pull it down from iCloud so it’s not Apple’s comment but mine. And yes they did know there was CleanMyMac installed.

Try to be objective and on point. This just elevates things even higher IMHO

You are welcome for the free assistance.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise

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