Too much unknown data in my MB pro

Hi everyone.

It's not often that i turn to the community but lately i'm scratching my head with this.

So, i have a MB pro, 1Go storage.

I use it mainly for work (design) and writing, with the occasional imovie project.

All of my data is stored either on my work computer or in the cloud, meaning i do not have a lot of stuff in this computer, more or less 150Go all media/files combined.

It's important to know that all my pro life is backed up between cloud and computers.

Now comes the problem:

it's been something like a year that i'm seeing storage space shrink like wood on fire. Right now i'm at almost 500Go in System Data with Time Machine disabled.

I've also disabled any backup from my tablet or my phone.

What's all this stuff?!? Even weirder, i've used 3d party soft to check what was inside those 500Go and see if i could erase some stuff and it couldn't either tell me what it was, and i just regained 15 Go by deleting cache i couldn't access before.

In a similar note, i'm at the point now with iMovie that after deleting the render files, when i open a new project it creates more than 250Go of data from a 8Go media.

What is going on?

• I've done a lot of tricks (TM etc) to lower the System Data to no avail.
• I know the Delete render files trick for iMovie, i'm just at a point where i can't open a project without having immediate 250Go created on the fly, maxing the storage capacity of my computer and precipitating an emergency shutdown of the software.
• I've tried to use an external hard drive in various manners with the iMovie library, which ended up slowering the computer a lot and making exports crazy long, often unsuccessful.