@flownty The short answer is, you can't free up space without deleting data. What you can do is go through the Downloads folder looking for old files to delete, for example installers for programs you've already installed or .zip files of content you've already extracted.
If that's not enough, the next step would be to uninstall a game currently on C and reinstall it on D. Sims 4 would load more slowly on D, since it's a mechanical drive, but the difference wouldn't be unmanageable. And the game alone is large enough that this would take care of the free storage problem all by itself, at least as things stand now.