Not amateurish at all, but good questions. I’m not confident enough to answer your second question, but on your first question: yes, you can use the hosting service you already have. Given that you are renting a VPS that is. It looks like Strato is calling this “hosting”. Don’t use their “WordPress hosting” package for this though, as it won’t allow you to install the instance software.
We’re using backstage in combination with openapi. The schema is documented in OpenAPI, but how services are connected is done via backstage, which crawls all repositories and puts it together to form nice graphs we can traverse easily