They started from the inside out. Moses and his sons washed their hands in the washstand. Moses could not enter the Mishkan because the cloud of G_d filled it. “When the cloud rose up, the children of Israel set out in all their journeys. And from then on, there was a cloud over it by day, and fire at night.” The fire is the menorah that never goes out.
I like the idea that the children could leave when the Mishkan was finished. In a sense you can't leave home until you have a home. What would leaving mean if you had no place where you had roots?
I have roots in a number of places. When I go to a new place to live, I can't really leave until I have settled.
At first the cloud covers the Mishkan. Later it is over it, but not covering it. It would seem in the hot weather the cloud would be a relief from the hot sun. It would show that G_d was compassionate.