600 GB storage is a massive data stored in documents / files , you can archive them .
Open system preferences > iCloud and uncheck the box of iCloud Drive a pop up window appears , showing to keep a copy of documents , and remove from the Mac .
( a ) if you had clicked on keep a copy , they will be archived in home folder that can be viewed by clicking on finder in the dock , take cursor on top menu bar > Go > Home > your user name > iCloud Drive ( Archive ) .
Again double click empty iCloud Drive box it will show setting up the box will get checked , the data is saved .
Then you can drag and drop the massive size documents see this https://support.apple.com/en-gb/guide/mac-help/mchlp1774/10.14/mac/10.14 to some other location https://support.apple.com/en-in/guide/mac-help/mchl1a02d711/mac
Or delete them https://support.apple.com/en-in/guide/mac-help/mchlp1093/mac
See some articles to reduce the data https://support.apple.com/en-in/HT204247
https://support.apple.com/en-in/guide/macbook-air/apdb468896e0/2019/mac/10.14.5
https://support.apple.com/en-in/HT206996
https://support.apple.com/en-in/guide/system-information/syspf5a64aa6/mac