"...a foot and a half of substrate" ???

I am confused. The minimum recommended for drainage gravel/leca/hydroballs/etc. is two and one half inches. The fluffy substrate layer on top, in which the plants grow should be the same. For an enclosure suitably large for a BP, I would suggest three inches of each, giving you six inches, total. Leaf litter, etc. goes on top of that.

Remember to plant sparingly - leave some room for the snake.

The bio-active part is more in the soil microbes and tiny critters than it is with the plants. Think of a successful compost pile - it is bio-active, but has nothing planted in it.