Substrate advice for bearded dragon bioactive enclosure

[Mightaswell]

Hi all!!
So I have this newly finished 4x2x2 bearded dragon enclosure that I need to fill with substrate. The inside dimensions are 46.5 inches x 22.5 x 22.5. I have a 8 inch deep wall on the front that I want 6 inches of substrate that is safe for bearded dragons and able to hold live plants. With an inch on top of oak woodchips. I've chosen spineless prickly pear to go around the walls and sides. I feel it's a great choice as it's safe for them to eat and survives in arid to semi arid environments. I'm hoping to make a semi arid bioactive enclosure because I want to add a mulberry bush inside as well.
I'm thinking if I mixed 100lbs of play sand to 100lbs of organic compost and 6 or so coco coir bricks hydrated all together I'll get a sandy loamy soil that I can pack a bit so it's not too loose. I would then cover that with oak wood chips to hold in the soils moisture.
So what do you guys think? I want to add isopods, nightcrawlers, springtails and whatever else for a cleanup crew. Any advice would help. Cant figure out how to upload pics or I'd post my enclosure.