I'd you're making it for the soil, I think you're better off just making charcoal. Properly pyrolising the wood takes a disproportionately large amount of effort for the return that you get.

I use a kiln made from sheets of corrugated iron to restrict the airflow and reflect heat in. I add more feedstock as the top layer turns to charcoal & then quench with water when done.

If I really wanted I could put a small inverted drum in the middle to make bio char.

As a soil amendment I focus on creating as much charcoal as I can with my time. After a 6hr burn I might get 1/2 a cubic metre of charcoal. The stuff is incredible for the soil and has very little cost. I just use fallen branches & sticks which don't need to cut up much.

I understand that you're learning & it's fascinating. I been there making TLUDs & such. The open corrugated iron kiln was a game changer.