Unfortunately, I don't think passive air flow through the greenhouse will be sufficient to keep things cool enough. I'm using 2 attic fans on a temperature controller in my greenhouse to move the air through it. The oscillating fans will only do so much. Your screened door will provide some relief but I don't think it will keep the heat down enough ... but I could be wrong about that. Dehumidifier - Your greenhouse likely isn't air tight enough for a dehumidifier to do much more than run up your electric bill. Essentially, you'll be trying to dehumidify the whole Long Island area.
Last, it looks like you have 15 plants or more ready to go in that space. That's too many. Your greenhouse at 8' x 12' is 96 ft square. Mine is 10' x 20' - 200 ft square. One time, I tried to grow a dozen plants in there. By fall, it was a jungle and I could not walk down the center of the greenhouse without damaging the plants.
I'm in Michigan. The night time temperatures from September on makes it difficult to grow good outdoor weed. On the other hand, I have zero issues growing inside where I can control all aspects of the environment. I spent my first year attempting photos in the ground. My second year, I ran fast flowering plants in fabric pots in the ground .... the last several years, I ran auto-flowers which I had my best luck with, but I still ended up dealing with some bud rot on my second summer run. I ran autos started indoors in April, moved them to the greenhouse in May and then started another round. I got 24 plants through it in 2 different runs but anything not finished after labor day ended up with bud rot.
My question for you is can you keep some of those plants in their fabric pots and grow them in the open air? You will actually have less humidity issues out in the open than you will inside your greenhouse. Next, I suggest you either dust off your battery powered leaf blower or buy one if you don't already have one. You'll need it come fall to dry off your plants EVERY morning.
This year, I'm running tomatoes and peppers only in my greenhouse ... no cannabis.