How can I effectively optimise System data on my MacBook running macOS Ventura?

It is generally a good computer practice to alway keep at least 20% to 25% of the Total Drive Capacity’s as Empty Space. Allowing the computer to drop below these guidelines may eventually, cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur.

The links below will help in identifying what is taking up space on the Internal Drive.

It will also provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

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

GrandPerspective 

How to delete Time Machine snapshots on your Mac.  

Often caused if the Time Machine Drive has not been attached  to the computer and TM Backup is set to run on a Schedule. 

TM Backup will make Snap Shots on the Internal Drive awaiting the TM Backup Drive to be attached. 

Only then,  will the Snaps Shots be transferred to the External Drive.

View APFS snapshots in Disk Utility on Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

If there a Suite of Adobe Applications  used on this computer ?

They may create some very large cache files that can be removed. Though, the Adobe cache files will be recreated as the Applications needs them.

https://helpx.adobe.com/ca/premiere-pro/kb/clear-cache.html

The same clearing of System Cache files can be achieved by booting into Safe Mode

They will be recreated as the System Requires 

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

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.

There are areas of the Operating System which we users have control over and that is the User Account Folder ( Home Folder )

All other areas are not accessible 

The System Data used to be called Other Category which is a potpourri of files which can include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

• Files created and modified by other user accounts on your Mac.

They can be located anywhere on your hard drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use this free app, GrandPerspective, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Some users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

Also, if you use Time Machine to backup your Mac launch Disk Utility and check your boot drive for local snapshots like in this screenshot:

I check it about once a month and remove all but the latest.

Additionally, have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Are you backing up to Time Machine? If the drive is not connected, it will make local snapshots. Those will be reported in System Data. However, the OS will delete them as necessary.

It may be your Spotlight index is corrupt. Anything it cannot categorize goes into the System Data category.

Rebuild the Spotlight index on your Mac - Apple Support

Reindex the drive and see if anything changes. Part of System Data may be your files.

I don’t own any Adobe apps, so I don’t know if they try to manage writing files on their own or use the OS to do so. If they do t ask the OS for space, it won’t get purged.

Judging from your image above

A 256 GB Drive Capacity with 243.74 GB used

In theory, the computer has about 13 GB of Space Remaining to work with

Would suggest using Disk Utilities to get a much more Accurate view of Used versus Empty Space

In either case, strongly suggest using the suggestions from both contributors to address this issue - Pronto.

Not to scare anyone, but is left unaddressed.

The machine may eventually cease to boot up at all

That would create yet another avoidable issue