If my 5th metatarsal break heals will it be more prone to breaking during activity in the future?

Maybe someone will . Disagree with me. I am not quoting any medical literature. I would think the bone strength will be as good as new. I think the mechanics of why it happened if not altered could bring you back to the same problem. I have had many patients with broken 5th mets in the last 16 years. Not one re-fractured their bone.
No. Fully healed fractures usually are not more prone to refracture.
No. There is no evidence that a broken bone heal to be stronger than it was before. Due to the process by which bones heal, there may be a brief period in the healing process when the fracture site is stronger than the surrounding bone. However, they later reach equal strength, and the fracture site is no more or less likely to break again.