Background looks great! I am not sure how I would hide the seams in the background. Might be able to get enough paint down in there to hide them some, but in all honesty once it is painted and planted you will be the only one who can se the seams in the background easily. Everyone else will look right past them

If it was my background I would probably be using either the foam coat from hotwire foam factory or unsanded tile grout colored with acrylic paint to do the base coat on the wall and then cover it in Drylock paint with various shades of tinting to it.

Short answer. there isn't really a wrong way to do it as long as you ensure the materials you use are nontoxic.

Paul