I was thinking about putting ventilation along the top of the back wall, pretty much the whole length of the wall (maybe stopping before the roosts?), 1 foot tall. Then make a wooden “awning” over the top to protect from rain. Possibly do it in two sections and install it with hinges so one or both can be closed temporarily if absolutely necessary for weather.
The easiest thing would be a top-hinged cover that you could prop open at different levels so that you'd never been shutting off your air circulation completely.
just needed to figure out airflow between the pole barn and timber
The warm air rising in the coop will create natural circulation. This illustration is from an article on cow barns, but the principle holds:
