(Not a Teacher)
I reworked your sentences. I'm not sure if I fully understood what you were trying to say, but I gave it my best shot:
"I strongly believe what constitutes good cinema is an entirely subjective issue. One movie may suit your tastes, but not another's."
"If you want a snack during the film, you are not allowed to pause it; you'll have to miss part of it to make a trip to the concession stand."
I'm honestly not sure if there any hard and fast rules on the use of dashes and semi-colons. To me, they represent a pause between two complete ideas that are closely related, if that makes any sense. A comma and conjunction don't seem appropriate here. It makes it sound like a run-on sentence. You could just put a period and it would be fine.
Student or Learner