Currently there isn’t a method to directly filter relations unless you use GraphQL or you construct your own custom query: https://strapi.io/documentation/developer-docs/latest/concepts/queries.html#custom-queries
You can customize the existing controllers or build your own (you can skip the service and simply execute the service type code in the controller instead)
For clarification if you are unsure how the chaining works in Strapi:
User Request => U&P Plugin => Route => Handler (Controller) => Service => Query => Database (via the ORM)