Let's get the apologies out of the way, if I offend it's only because I want you to succeed.
Our grows are very similar, both outside of the house, both 8 x 14 (+/-) yard barns as we call here in the midwest. Cooled by window unit's that are both 6000BTU units. Lots and lots of similarities, even looks like the beginnings of a lung room for you veg plants. Nice, Very Nice!
Now, like I mentioned we both grow in yard barns, outside of that nice air-conditioned home where all the other farmers get to grow. We grow outside where temps and humidity vary greatly. Your ventilation design is based off of an indoor set-up, not a yard barn. 2 completely different ways of venting a grow room.
Let's start with the dehumidifier:
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The window AC unit and your dehumidifier are basically the same thing with 2 small differences. DeHumidifier only has one fan and it has a bucket. How they work is by compressing a gas (let's say freon, but it isn't) that compressed gas gets cold on one side and very hot on the other side of the compressor. Room air from an ac only passes through the evaporator coils (cold) and returns to the room colder and drier. Room air from a dehu passes through both coils, 1st coil removes moisture which then passes over the heated coils to cool them before returning to the room warmer and drier. There's only 2 vents on the dehu, all the air your running through the unit is being blown out the wall. The only, and I mean only, thing your dehu is doing is adding heat to a moist room. Unplug it and store it away until next fall. The window unit will handle all the excess moisture in the room. All the air that unit was blowing out the wall had to be replaced by outside air being drawn in. Hot, moist air constantly being pulled in.
Your intake fan, unplug it as well. There is no need and besides the AC needs time to cool the air. The exhaust fan, yea you guessed it, unplug that bad boy as well. No need.
Now a little math to back up those last couple of unplugs. Yard barns let's say 8 feet wide, 12 feet long, 8 feet tall. That's 768 cubic ft. A 4" fan is going to be rated real damn close to 200cfm (cubic feet per minute), 6" is 325cfm. So a 4" fan can empty the room in about 4 minutes, 6" in a little over 2 minutes. Now add in the DeHu being vented outside (200cfm guessing) and the air inside the grow room is being exchanged every 3 minutes give or take and there is no AC that can possibly keep up with that (that we can afford).
So about all that's left plugged in are the lights and AC, use the fan filter combo to scrub/circulate the air but not discharge it outside. It's cool clean air now. Take your intake fan and vent it to the exhaust port in the wall. Put a thermostat on it (mine's @ 84) and it's now your back-up exhaust incase the ac craps out. Your thinking about CO2 probably, remember your in a yard barn with plenty of fresh air.
Growing in a yard barn takes a whole different set of rules to be successful. I have 3 ventilation configurations, winter summer and inbetween. I realize everything I just typed is contrary to what folks do, but most folks don't grow in a barn
jj