System storage is REALLY big

Hi, I have an iMac 27 with 4 TB Fusion drive storage running High Sierra 10.13.5. I have Time machine running, and it seems to be working fine. My problem is that system storage seems to have eaten all my free storage. It sits on 1.52 TB which i think is a lot. Even though I do have many videos, pictures and music, this seems absurd to me.

I have tried reading similar post, but none of the fixes seemed to change anything. I've tried turning turning automatic TM backup off and restarting. I've tried reindexing spotlight. I have also tried searching for local TM snapshots but "sudo tmutil listlocalsnapshots /" comes up with nothing.

System seems a but unstable with disk running a lot and sudden crashes.

Can anyone please help me?

(My understanding of technical stuff is limited so it would be great if you could explain suggestions throughly. Thx!)