Once you insert the TextBox from a Building Block, you can easily move it to where you want it. It will start at the position it was in when you saved it originally as a Building Block.

Your proposed layout has the text box in front of text. Are you sure that you want to occlude text in your document level?

While you can do what you want using a macro, you are reinventing the wheel.
Yes, you can use a macro to insert AutoText or another building block. You can also simply attach a keyboard shortcut to the Building Block.

Even though it is dated, I strongly recommend that you review Word for Wordperfect Users. Trying to do things in Word the way you would in WP makes things much harder.