In wallhangings, I usually use a wool on top and an 80/20 on the bottom. That will give you definition without being puffy, so long as you have adequate quilting.
Make sure the quilter is aware of the Steam a Seam. I would make a small piece (16 x 16 or so ) for them to practice on that had the same depth of collaging as my main piece does and batted and backed the same way, unless they are very familiar with collage and working with fused fabrics.
For my art quilts, I find Steam a Seam 2 Lite is usually my choice, just for the reason you mentioned. It is plenty strong enough but a lot lighter, IMO.
Watson