One approach could be to first filter for all backlinks, then explicitly exclude any properties that also contain a backlink. This works well if you only have a limited number of properties that could contain a backlink — you can simply create a separate filter for each one.
If you want to exclude all such properties with a single filter, you could try to expose the frontmatter using file.properties and use a formula to exclude backlinks from there. The values() function may also be helpful in this case. But I have never tried this.