If the survival rate is constant, the survival plot would be a straight line (slope = survival rate = a constant) - right? So, the non-parametric Kaplan-Meier model will work (as in "Keep It Simple").
Dear Prof. Daniele Porcheddu , I would recommend this book: Cummings, P. (2019). Analysis of incidence rates. CRC Press. Thanks. Best Regards Salvatore Saiu