How to vent to prevent mold - hexagon house

I have a little bit of a unique issue, I bought a log cabin in northern West Virginia a couple of years ago. The cabin is unique in that it is hexagon-shaped with each of the six sides spanning 20 feet long along the outside walls. There is no attic as the ceiling vaults to a single high point at the center, and thus the cabin is one big open room (almost 1,000 square feet). The original shingled roof was leaking when we bought the place back in 2017, so we had a new standing seam metal roof put on. The place is vacant most of the time (we get up there about every other weekend) and we’ve been battling mold on the ceiling for about a year now, the ceiling is wood shiplap. The original shingled roof had small vents on each of the six sides that vented into the joist space, but the professional roofer we hired to put on the metal roof was very adamant that venting wasn’t needed as there’s no attic.

I bought a plug-in dehumidifier a few months ago, but it doesn’t seem to be keeping the mold back. Last weekend the humidity level on the floor with the dehumidifier was 60% while the humidity level at the top of the pitch was 90%. The ceiling was actually wet to the touch. I’m really in need of some good advice here.

I was thinking about having a solar-powered attic fan installed toward the pitch, but the cabin would be open to the outside. Plus, I can't seem to find any examples of such a product on my type of roof.

I may not be doing a good job in describing the place either, but can provide some pics for perspective. Please help if you can.