6 GPUs may be the max this case will support utilizing just the stock GPU fans. 7 may make them too cramped to allow adequate airflow.
So with 6 GPUs the temps were getting into the low 80's even with the GPU fans bumped up to 90%. It was time to figure out a way to cool the GPUs some more so I decided to try adding auxiliary fans.
It blew me away how much this helped!
(2) 200mm fans and GPU fans back down to 80%:
From left to right GPU#s: 2 0 3 4 5 1
These 200mm fans fit almost perfect. I could drill holes to secure them but will most likely just use zip ties. If I ever make another case I may try to fit them inside the space between the front/back (depending on how you look at it) of the case and the GPUs.
Yes, they are blowing air at the GPU exhaust vents since these particular models mostly expel air the other way. They are plugged into motherboard fan connectors and are not running at full speed. Very happy with the results and surprised that it helped as much as it did.