I do not wait between spraying and and smoothing. I understand that may be needed when glue basting. I am only addressing spray baste that dries almost instantly unless it is applied too heavily.
Step 1 - press my backing to get out any fold lines.
Step 2 - I lay out the backing on a table and use clamps to hold it smooth. (I have access to moveable tables at my Senior Center. It is so worth the membership fee to have these tables for sandwiching my quilts.)
Step 3 - Lay on batting ( I use Warm and Natural. I have never used a poly batting.)
Step 4 - Lay on top This allows me to be sure back, bat, and top fit and have sufficient extra fabric all around.
Step 5 - I fold back about 1/2 of the quilt spray the back, smooth on the batting, spray the bat and smooth the top
Step 6 - Repeat with the other half.
Step 7 - Smooth with your hand to detect any folds and correct them now while the bat is still help smooth.
Step 8 - Remove the clamps that are still holding the back. Quilt away!
As suggested above, particularly with a quilt the size you have, do Steps 5 & 6 in smaller segments. Better yet "phone a friend" to help you move those large amounts of fabric and to smooth them with your hand as you go along.
I wish you success in reworking your quilt sandwich.