Deleting files and empty trash does not increase free space
The disk utility error is the main issue you face.


Make sure your important files are backed up.
Boot to internet recovery or an external drive so you can re-run Disk Utility not from the booted container. If you can't clear the errors or flush all the snapshots and get a clean run - it's time to erase and reinstall.


Also - you'll want to remove snapshots from all of your volumes - not just /System. find all your volumes on the synthesized volume and check them with diskutil instead of tmutil

diskutil list
diskutil apfs listsnapshots disk1s1


As for the end result - you'll get the most space back deleting the apfs snapshots against /System/Volumes/Data as you mentioned in your post:

diskutil apfs listsnapshots /System/Volumes/Data
diskutil apfs deleteSnapshot disk1s1 -name com.apple.TimeMachine.2019-12-27-124916.local