Mac mini M4: Excess disk space after Migration Assistant

There might be an issue with local Time Machine snapshots and system backups. Check About This Mac > Storage. After turning off Time Machine and restarting, space should be freed up.

Two cents here

Reducing System/Volume/Data is a common question. 

1 -  System data taking too much in MacOS Sono… - Apple Community

2 - Time Machine Local Snapshot won't delete - Apple Community

3 - Over 60% storage blocked by System Data - Apple Community

How to free up ‘System Data’ and other storage on your Mac from a fellow colleague  @ neuroanatomist

Use another application to see where space is being used  Storeograph  on the Apple Apps Store 

View APFS snapshots in Disk Utility on Mac

Suggest getting an External SSD Drive and start moving your Pictures, Videos, Music and any other large files you have control over, OFF the Internal drive and Onto the External

Understanding iCloud Drive from a well written User Tip from @ Richard.Taylor

There are  two effective ways to remedy this issue:

1. Quick Fix Actions:

For Apple Silicon computers, use Disk Utility to erase a Mac.

For Apple Intel computers, use Disk Utility to erase an Intel-based Mac, then reinstall macOS.

Always make a Time Machine backup before proceeding.

 Migrate only the user account, not the entire system.

   Reinstall only the necessary applications from the Apple App Store or directly from the developers.

It's wise to keep a minimum of 100GB of free space at all times to facilitate optimal system and application performance. The following is how to move large files and folders to an external SSD and operate from there:

1 - get an external SSD drive and format it APFS.  (I recommend OWC (MacSales.com))..

2 - copy the Pictures, Movies, and Music folders to the external SSD.

3 - make sure the libraries in those folders work as expected on the external SSD.

4 - once confirmed the libraries are working OK on the SSD delete the contents of the Pictures, Movies an Music folders.  DO NOT delete the folders themselves.

5 - rename the folders on the SSD to Pictures-2, Music-2 and Movies-2.

6 - remove the original folders from the Finder's sidebar and replace them with the -2 folders by dragging them from the SSD into the sidebar of an open Finder window.

I have a Mac Mini M4 and use the above to keep lots of free space.

Here are a few related discussions which may be useful.

Free vs available disk space huge differe… - Apple Community

My hard drive is showing more usage than … - Apple Community

System Setting > General > Storage is not totally reliable