In my media library all images are listed as unattached so I have no idea how else to delete unused files there.

I’m not familiar with the WPBakery page builder plugin (it’s a commercial product often included with commercial themes), but from what you’re describing, it appears this plugin doesn’t use the native WordPress file attachment mechanism, making WordPress unaware that your images have been attached to content created by the plugin.

That would also explain why you wouldn’t find any “plugins or solutions directly related to this problem”.

(It’s for these reasons that I personally try to stay with WordPress’s native tools and freely-distributed themes and plugins as much as possible, even if they’re not the best options in their category)

I’ll advise you to reach out to the WPBakery plugin’s developers for help in separating your attached vs unattached media… as there’s not much we can do to help you here without having access to the plugin at the centre of this.

Good luck!