Hello and thank you for reaching out to us again!
Here some general information of the declaration of custody:
The declaration of custody (Sorgerechtserklärung) is a declaration of intent by the parents to the youth welfare office or a notary that they will take joint custody of the child. It is necessary if the parents are unmarried at birth. Because then the mother has sole custody. If both parents are already married when the child is born, they automatically have joint custody. In this case, a declaration of custody is not necessary.
I am not an expert on family law, but it sounds to me that you might have stated the joint custody already since you "have Vaterschaft and Sorgerechtserklärung". Since there seem to be some apects unclear, I recommend you to get in touch with a refugee/migration counselling office (you may use this search engine to find a service in your area). Together you could check your documents and they can also support you with the naturalisation process.
In any case, please also don't hesitate to get back to us with any further questions.
All the best,
Meike