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.