System Data 213 GB / 240 GB on my internal drive (macOS High Sierra 10.13.6)

Some of the data.....especially if you have deleted large amounts in an attempt to free up space is likely still contained within the hidden APFS backup snapshots from Time Machine and Carbon Copy Cloner. These APFS snapshots are usually deleted automatically at some point. For TM snapshots, it is usually about every 24 hours, I don't know about CCC or whether CCC may provide configuration options for setting how long APFS backup snapshots are retained.

View APFS snapshots in Disk Utility on Mac - Apple Support

In addition you should enable TRIM on the SSD to help with SSD performance by using the following Terminal command:

sudo  trimforce  enable

This command will prompt you for your admin password. Nothing will appear on the screen as you type the password, so press the "Return" key to submit the password.

You definitely need to keep at least 20GB+ of Free storage at all times for the normal basic operation of macOS. For many workloads you may need to have a lot more Free space. The Free storage space value is only shown in Disk Utility and the System Profiler....ignore the "Available" storage value shown every where else since it is very misleading.

And @PRP_53 is absolutely correct regarding CMM and any similar types of apps. Plus it appears you don't have a large enough SSD for your boot disk. You could easily increase the storage by creating a Fusion Drive setup or by replacing the SSD with a larger SSD. Surprisingly some of the larger hard drives are now much faster...some up to over 150MB/s to 200MB/s which would be fast enough for a SATA II interface used by this 2010 Mac Pro (the interface maxes out at about 250MB/s), but I don't know what drive sizes this Mac Pro can support or whether the smaller hard drives have those speed improvements from using multiple drive heads.

We see, the user ( you ) has resorted to and possible believing "  CleanMyMac " may have offered an easy solution to lack of space on the drive.

Sorry to say, it may have done just the opposite and made a bad situation even worse.

The KINGSTON SV300S37A240G 240.06 GB (Solid State - TRIM: No)

The drive has only a finite amount of usable space and once it is full it is full

You may consider you options carefully when moving forwards

Get a much larger SDD drive for this machine and Start Over from Scratch

CleanMyMac , aka “ BrickMyMac “

New macOS Malware "Cthulhu Stealer" Targets Apple Users' Data

Un-Like Microsoft Windows, which does have “ Disk Cleanup “ and “ Defrag “ as part of their Operating System 

Apple does not, nor are they required 

As a last resort you can erase the drive and reinstall the os then migrate your files. I have found that this process lowers the system data quite a bit.

I think you are missing the point

In case it was missed earlier which can happen

" Get a much larger SDD drive for this machine "

An aside to this ongoing issue

When " My Go To " hardware experts weights in

I do pay special attention to their offering