Beautifully pieced. I'm impressed you can actually fit a King sized quilt into your DSM. I wouldn't be able to fit one into mine.
I would use 2 colors of thread & 2 different designs. I'd use a pale grey in the pink and blue area & a cream colored thread in the center. I think I would maybe try either ribbon candy or Leah Day's "Wiggly Pasta" design for the pink & blue knots with a wiggly circle where they meet; and then would just do SITD plus straight lines (all one direction) in the centers with cream thread. A 50wt thread will help it blend in with the fabric more than a heavier weight.
The main thing you have to keep in mind if you go with a variegated is to make sure no part of the variegation is an exact match to your fabric or that part of the design will completely disappear.