Love that you’re planning way ahead!
I don’t have answers for you about substrate with worms.
I do believe that you’d be wasting your time with diatomaceous earth in your substrate as it’s rendered useless when it gets wet. It’s also a drying agent so may cause problems with their feet over time.
As for plants and height, well...
Now, take all of what I’m saying with a grain of salt because I’ve only had quail for a month, lol, so this still may wind up being no good. xD
I have a small wooden house that I built for them to keep their food and water in, and a cage that was originally meant as a rabbit run. It’s 8’ long, 24” wide, and 27” tall. Gasp! Lol. So far, all my quail have done when out in the run portion, is hop and flap and get maybe 12” off the ground - and those suckers can fly if they want to. They constantly were flying out of their brooder and flying up to places that made them hard to retrieve.

Now,the location I placed the cage and their house in is beside my house, with electric netting around the duck house, chicken coop, their runs, and this corner where I tucked the quail cage. No one has tried to flush as of yet, probably because they have 2 shrubs in there and a couple fabric plant pots to go hide in. They get startled, they scurry under the shrubs or into the plant pots.

To be on the safe side, though, I did put up bird netting as a false ceiling to lessen the impact if they do flush, and covered the back and top of the entire thing with a tarp to provide shade and protection from wind. It doubles as a blind so they don’t necessarily see the ravens etc flying over. They’re just on bare ground, I have sandy soil that drains fast so it’s not a quagmire. I’ll probably wind up putting down leaf litter in the fall and doing some version of deep litter through the winter to help them stay warm and insulated from the ground.

you won’t have that issue if you have them inside a building though!