Disappearing storage on iMac—PLEASE HELP

We have an iMac with 3TB of storage running on Big Sur, and 2 years ago only 1TB was used up. Most of our storage comprises photos, videos, and some documents, as we have this Mac for personal use.

Over the last year or so we’ve noticed the Documents file is somehow over 2TB, and many of our videos and photos have been unintentionally duplicated many times. There are multiple copies of many songs in iTunes as well. We only have about 200GB of space left. Our 8TB external hard drive purchased about 2 years ago is nearly full.

We’ve tried deleting files we no longer need, emptying the trash, etc., but it almost feels like once we make some space, it’s gone again. We’ve stopped backing up our iPhones to the Mac because there isn’t enough room.

Thanks in advance!