System Data taking too much space

The system data is storage used by the system that is used temporarily for caches to speed up the performance. How much "system data" will be used, depends on the state of of your Mac. Right after a system upgrade the system will need more System data and use more storage, if there is plenty of free storage, but release it again, when the storage gets low. It is fully automatic, and usually we can ignore the used "system data". We cannot manage this storage. See:

See used and available storage space on your Mac - Apple Support

"

• • macOS: Contains macOS system applications and files. Examples include Mail, Terminal, and Calculator.
  • 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. You can manage your data that falls outside the other categories using Finder or the 3rd party apps that created it."

The sad truth of the matter is, the user purchased the smallest sized Internal Drive at the time it was New.

Agree, this size of Internal Drive should have never been an option but it is what it is and the user will be forever diligent on what is kept on the Internal Drive

This model shipped with either a 128 GB or 256 GB SSD by default, but it could be upgraded at the time of purchase to 512 GB or 1 TB at additional cost.

This may require a purchase of an External Drive and archive unneeded Photos, Music, Videos etc onto the External Drive.

Rebuild the Spotlight index on your Mac

Or Force a Reindex via Terminal Command “ sudo tmutil -E / “ without Quotation Marks 

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.