Would love to see a video on your channel of how you wire.
Its a area I'm not fully getting my head round.
Be cool to see how you do it.

There´s plenty of that all over the net
Mauro has a great lesson in Bonsai Empire course too.
But I might film some wiring action in some of the future videos for you

I am curious about watering a tree placed on a flat rock. How do you keep the soil from washing away? Do you water more often than trees in pots?
Not watering, but what do you do about roots growing out of the soil? Just cut them ofg?

You use KETO for it to keep it in place.
I cover the whole substrate with it, then apply moss.
And water and repot as you do with a tree in a pot.

When a tree in a pot grows its roots out to the edge of the dirt there is a pot there and the roots bend around and stay in the dirt because of the pot. On the rock here it looks like the roots would keep going straight and shoot out of the substrate and moss. Do you trim constantly?

Well, I don´t, on this one, there is still plenty of space for the roots inside.
But when the roots come out from under the substrate on my ficuses, I cut them every now and then.