I’m sorry to say it but … what you’re offering isn’t a real trial.
Ok you want the credit card in advance (not nice, but it often happens).
But if you offering me 15 days of free trial, why do you have to cancel them immediately if I decide to cancel in advance the automatic renewal?
I ALWAYS prefer to cancel the automatic renewal and decide later if I want to buy or not. It never happened to me that this meant ending the trial period…