Also, Review your retention policy to see how long you actually have to keep each category of files.
If you need to delete things the hard/cheap(?) way, instead of expanding your storage or integrating with outside-of-NetSuite storage, I'm not sure the best approach but I thought of an idea of downloading all the files (folders can be grabbed as a ZIP file to save time) to a big hard drive and running a local LLM on the whole thing to report to you what all the files seem to be about. Then you can delete the ones you don't need from the Filecabinet (if any are found to be no longer needed), if that complies with the company's retention policy so you don't delete things you wish you hadn't.
That way you have a backup on the big hard drive (and anywhere else you copied that to). Be careful not to lose anything, AND not to send or copy it to anywhere or via any route that would expose sensitive data.
For the immediate solution though it's probably better to expand or integrate with outside-of-netsuite storage.