Once the home was roughed in for plumbing and electrical, we added the insulation. The hard part with a spray in foam and metal studs is that the studs can become bowed and uneven for placing MDF or shiplap on the walls and ceilings. That is why we added the drywall before adding the MDF. We wanted to ensure the walls and ceiling were plumb because I’ve seen plenty of cases of MDF looking like waves and I’m not a fan of that look!
We decided on the MDF because we know that it handles movement much better than just drywall alone. We installed drywall in several tiny homes on wheels that we built (not our idea) because the customer insisted on it. The issue with drywall in a home that moves is it will crack at some point. Granted, it’s not difficult to patch up, but it’s one more thing to do once the home has been delivered.