The best and safest option is to do the migration without deleting anything on the old Mac, then immediately perform a Time Machine backup on the new Mac using a brand new external drive. After the Time Machine backup completes (it may take days), then you can delete files on the new Mac which you think you don't want or need. If you find out later you deleted something important, then you can retrieve it from the Time Machine backup. Keep in mind though, that eventually those files will be removed from the Time Machine backup to make more room once the backup drive becomes full.
Back up your Mac with Time Machine - Apple Support
Restore items backed up with Time Machine on Mac - Apple Support
Unfortunately we don't know which files are important to you either. It really helps if you develop a good "filing" system so that you know exactly where all your important files are stored which means keeping track of where you save files when using the "Save" option in various apps. When in doubt keep it.
Anything located in your home user folder(s), Desktop, Documents, Downloads, Pictures, Music, Videos was placed there by you at some point. You can delete any thing in these folders that you don't want, but be careful with the Pictures, Music, Videos folders since they are controlled by the macOS apps -- you should use the macOS apps such as Photos, Music, etc. to manage items in those folders.
The hidden Library folder in your user account will contain your custom user account settings and custom app settings. This is a very complicated folder so unless you know what you are doing, it is best to just leave it alone.
With recent versions of macOS users are no longer allowed to modify the macOS system files/areas so there is nothing to worry about there. If you follow my recommendation to manually reinstall your third party apps, then just leave the "Applications" option unchecked when migrating so nothing from the "Applications" folder will be transferred.
You should always have frequent and regular backups of your computer and any external medial which contains important and unique data. It is impossible to recover accidentally deleted files from an SSD after the Trash has been emptied plus an SSD can fail at any time without any warning signs (even a brand new SSD). In addition it is impossible to recover data from any 2018+ Mac if there is a hardware failure of the Logic Board which contains the integrated and hardware encrypted SSD.
