It turned out that the whole setup get much more flexible and the loaders and persisters much more reusable, when separated from the service actually transforming received data into the agreed on format to be used to communicate between loader and persistor.
It was kind of agreed on that it is a good idea to use immutable ValueObjects as the container to transfer information back and forth.
Using doctrine entities will make a persister even simpler.
It turned out that the whole setup get much more flexible and the loaders and persisters much more reusable, when separated from the service actually transforming received data into the agreed on format to be used to communicate between loader and persistor.