Hi nazbrok;
You don't have to put all the server side logic in the EntityServerSaveInterceptor class.
For example, in this particular use case, you can create a separate utility class with a utility method that will create the "many other entities" that you require before saving.
And then, in the ExecuteSave override, you can call this utility method to create those other entities and then call base.ExecuteSave().
My point is that you can separate the logic in a separate class but you have to call the logic within the EntityServerSaveInterceptor class.
Hope this helps.