I try not to plan my sentences like you said because I find that makes it worse. Another thing I do is substitute words I know I’m gonna stutter on for other words, “what day is it?” Turns Into “is it Tuesday or Wednesday?” Sometimes talking really fast helps for me. If I do plan my sentence, I won’t say what I’ve planned out. Instead of “I’m here to pick up my package “ I’ll say something like “I was just checking if my package was here”