Keep my garage moisture free

Your smaller building is similar to how I may do mine. Did you cut your foam boards to fit between the square tubing? The only ventilation mine has is where it isn’t air tight, I’ll be fixing that before I insulate it.
The previous owner did everything on the existing building. Other than it being really hot during the warm weather it's fine. It isn't air tight and was built in early 2000's for RV storage. I use half for my work shop and store my Sportfury in the other.