Fighting the same problems in an attic. My grow had initially been planned to finish much sooner, but was delayed as I kept killing my seeds.
I did note that you are still on 18/6, I switched to 12/12, not preferred but the average temps did drop a bit. That maybe a consideration if you are growing autos. I also added a homemade Co2 generator so they can deal with the higher temps. Raising the lights and dimming may also help. I wish my highs were 85F......cooler weather begins tomorrow and into the next couple of weeks.
Hope that is helpful.
Best of luck and curious if there are other options for you!