If you're looking for ontologies:
But there's maybe 5k ontologies, how do you hope to overview them and learn about them? You should start from a specific (narrow) problem domain.
Are these required and recommended properties only a thing which is understandable for the Google search algorithms to work well?
Yes. Follow additional guidelines in the documentation for your specific type. E.g., for Movie: required properties are name and image.
See also https://github.com/schemaorg/schemaorg/issues/1715.
Your question shows <> which is not a valid property or wildcard.
You cannot use property variables (wildcards) in property paths.
So you need to understand the structure of how dct:subject and skos:broader are used, and exploit that. I happen to know that
dct:subject links pages to categories
skos:broader makes the category hierarchy
I should warn ...