Do a test with a practice sandwich using all the fabrics in the quilt. Then you can see up close which thread would look the best. So many times I have done this and changed my mind about what I was so sure about ten mins. before. You must see it and audition.
And that is such stunning work. I know your grandma would be proud. I didn't know my grandmother quilted until I started to 19 years after her death. I feel like I missed out on something special.