Wow. Glad to find this thread.
I started growing petiolaris group last year, but I've had some failures.
D.dilatato-petiolaris and D.lanata (maybe D.derbyensis) good multiply many times. D. paradoxa, D. falconeri remain small or death.
You're doing great, I think I'm probably wrong.
My petiolaris box is 12 inch aquarium tank that is much shorter than yours, and the LED array light is directly exposed. I don't use a glass cover for the tank, and there is a gap for the LED array. It is very hot inside the box.
In Japan, planting in Sphagnum moss is mainstream, so Japanese hobbyists prefer this. But they smell rotten.
I don't use a fan and a heater.
Looking at your post, it seems like you're using tall pots and tall boxes. Is this the root length of the petiolaris group, or a reproduction of the local soil?
I recently got D x (broomensis x paradoxa) and D. caduca so I don't want to let this one fail.