As the others said, that exhaust is doing nothing for you right now. Your mini split is not bringing air into your room. It's simply cooling the air already in your room, which is why people prefer mini splits when it comes to sealed rooms. You should either just bite the bullet and use co2, or create a passive intake somewhere in the room to alleviate the negative pressure and bring in fresh air. However, doing this will result in you blowing your cold air out of your room, which is a waste of money. You could always set up a co2 system and keep it at a medium-low setting, like 500 or 600 ppm.
