My raised beds are 12” deep, and I use pure horse manure. The only other fertilizer I use is the rock phosphate. If your soil is anything like mine (caliche) you’ll have a high soil pH and will need amendments. If you have a deep soil base, you may just try adding the amendments to the soil, and checking it for pH. Many plants seem to like it when the pH is 6.5-7.0.
The big issue is keeping it there. You shouldn’t be going wrong with compost. Make sure you add a lot of mulch to keep the rootball moist and cool.
Bok Choi question...
Do you tie the heads/top leaves of your bok choi near harvest time to keep them from “spreading”? If you do tie them, what do you use?
I read that raffia or vines was good, but thought strips of ladies’ nylon hose might work well..