One thing I didn’t see you list was fan(s) to circulate the air within the space. Since you have limited free space, you might consider hanging some oscillating fans upside down, like I talk about in this thread:
Besides helping your plants grow stronger stems, fans will also help circulate your cooled/heated air and moisturized air when the humidifier is running.
Are you willing to cut a hole in your floor? If so, you could put an inlet vent on the opposite end of where your exhaust vent is and have some crossflow. It would pull air in through your basement or the space between your floor and the ceiling below. You can get filters for them, too.