System Data clogging up me hard drive

Hello friends.

Mail would not start as it said there was not enough room on the HD. My Mac Pro 2019 hard drive shows 217 - 233 GB of System Data on my 250GB hard drive. I have fewer than 20 photos and docs on the drive. Everything else is on external drives.

To free up space, I cleared my deleted mail and trash, deleted my iPhone back up, and checked TM snapshots.

How do I go about deleting files that are not needed?