Thank you for your question.
Phimosis as a diagnosis is made by examination and if there are no symptoms or discomfort during masturbation or when the penis is erect I'm wondering if the diagnosis is correct. You should be able to withdraw the foreskin easily when the penis is erect and clean under it when the penis is flaccid. If you are unable to do so then it is likely that the problem can worsen gradually.
It would be better to consult a surgeon / urologist who can guide you. The correct surgery is called circumcision. Calling it prepuce plasty is misleading because it implies that the prepuce remains intact which is not the case.
hope this helps