If your outside humidity is high then there's not much you can do but use a dehumidifier or the AC to get it down but if you have a lot of air flow thru the space then that won't work either. I have the opposite problem with really low RH most of the year. Thunderstorms today and the RH in the mancave here is just 40%.
If I can scrounge up the coin I'm going to get a portable AC unit and run a sealed room. Then I can keep the environment right where I want it. Have a CO2 controller and solenoid but need to get a tank and that's around $250 up front. $100/yr for the tank rental.
Just need a winning lotto ticket.