You’re welcome! (Sounds a little redundant, I know, but we mean it.)
I’m working on that very step right now. The sand cushions each firebrick and keeps it level. The herringbone pattern does a number of good things: if a brick were to rise, a pizza peel wouldn’t get caught on it; a more subtle benefit is that the horizontal force on each row of bricks does not ever go linearly from one side of the frame to the other. With firebrick reaching their heat potential, that means you’re not setting up a row of little jacks pushing outward.
Any gaps get filled up quickly with the ash from your curing fires; ashes don’t crack, and they let the brick move just enough to relieve stress. I believe that @BrickWood used to recommend spreading all-purpose flour over the hearth and working it into the small gaps, but it turns out the ashes from your fire work better for the purpose.
Having seen your inspiration photo on another thread, I know this is a dream working its way out. Bucket lists are a good thing, and may that bucket be a long way down the road for you.