Thank you for your answer my3bikaht.
When I do id(self.base_model) and id(self.m1.base_model)…id(self.mn.base_model) I get the same value. To my understanding, when they have the same id value, they are just a reference to the same object.
In any case, do you know any way in which I could prevent having multiple instances, by just having a single reference to the same object?