What is the actual ventialtion requirement? i know fungi produce CO2, so you need to ventilate a bit.
why don't you just use recirculation with economizer and cool air if it goes above 30°F, and heat it if it goes below 20°C, assuming that is a good mushroom range. If possible, use economizer.
for cooling I see the issue that you fight the need for high moisture. So when you cool the air in AHU, you dehumidify. This of course contradicts your goal of moisture. You could look in some radiant cooling? Especially since the cooling load likely isn't high.