System data over 500GB on 1TB volume on my MacBook Pro

Time Machine makes local snapshots and stores them on the boot drive. They can add up over time. See the following:

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

That is controlled by the system. You can check your snapshots in disk utility and remove to get some space or you can boot into recovery, erase the drive and migrate files back which usually lowers that number. Aside from that you can review this:

Free up storage space on Mac - Apple Support

Your Mac makes snapshots as a backup. These snapshots take up storage space. They can be seen in disk utilities in the data volume. You can remove some or all of these if they get too large. The system data can be reset by erasing your drive, reinstalling the OS and then migrate from your backup all your files and data. I have done this on several Macs and it usually reduces the system data, however, over time it will start to once again grow.

View APFS snapshots in Disk Utility on Mac - Apple Support