System Data Storage

It may thank a day or two for the Operating System to free up the space you have created by deleting what you have deleted. This I am told is normal.

Outside of the above >>

Rebuild the Spotlight index on your Mac This will give a more true picture of the space used on this computer

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

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

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.

because I wanted to hopefully solve this storage issue first.

Is it actually an issue? The OS moves data in and out of memory to disk and builds and destroys caches. The System Data will expand and contract as these things happen. When you delete files from an SSD, the storage space is not immediately reclaimed. It may take a day to reset the SSD storage to allow writing to those sectors.

The System Data is also a catch-all for things Spotlight could not identify as one of the other categories. It could just be your Spotlight index isn't complete or is missing information. I don't see your System Data being excessively large.

I would re-index Spotlight to see if it is an erroneous display.

If it is still large, use OmniDiskSweeper or GrandPerspective to see if you can find any large chunks of data that could possibly explain the large size.

This computer has been upGraded to newer versions of macOS exactly how many times in the lifespan of this computer.

In simple terms, can the user remember what version of macOS was originally installed on this computer when it was New and just out of the Box.

Illustration, MacBook Air from early 2015 qualify to run Monterey.

In 2016 OS X Sierra version 10.12 was released.

Then there was High Sierra, Mojave, Catalina, Big Sur and Monterey

In theory, this computer may have been upgraded 6 times in its' life and each time there has been an upGrade stuff has accumulated until Now.

As my good friend has offered a Terminal Command ( suggest following the advise ) which basically is covered How to delete Time Machine snapshots on your Mac only without the extra reading from that link

Generally, the files in the System storage can be anything that cannot be categorized into app, movie, picture, music or document, such as:

1. Essential macOS operating system files;

2. Time Machine snapshot backups;

3. System log files and cache;

4. Cache from Browsers, Mail, photos and the third-party apps;

5. Old macOS updates;

6. Trash data and junk files.

Sometimes your Mac builds up an unnecessary amount of System files due to a bug, but most of the time it’s a normal part of how the operating system works. However, you can follow the below methods to manually reduce system storage on your Mac:

1. Empty Trash

2. Manage Time Machine Backup Snapshots

3. Optimize your Storage

4. Remove iOS Backups

5. Delete Cache Files

6. Update macOS

Hope it helps!

You may have some Time Machine snapshots.

What is the output of the following command in Terminal:

tmutil listlocalsnapshots /