Preheat oven to 375°F. Mash 1 cup of the blueberries in a medium saucepan; stir in tapioca, lemon juice, and salt until tapioca dissolves. Stir in 1 cup of the blueberries and 1⁄2 cup of the sugar, and place over medium. Cook, stirring often, until blueberries begin to burst and mixture begins to thicken, 5 to 7 minutes. Remove from heat, and let cool 5 minutes. Place thyme and remaining 5 cups blueberries and 1⁄2 cup sugar in a large bowl; stir in cooked blueberry mixture. Set aside.

Unroll 1 piecrust, and fit into a lightly greased (with cooking spray) 9-inch pie plate; trim edges to lip of pie plate. Unroll a second piecrust on a very lightly floured work surface. Using a 1 1⁄2-inch round cutter (or a paring knife), cut out a circle in the center of top piecrust. Using a paring knife, cut 6 slits around the circle for steam to escape.

Spoon blueberry mixture into prepared piecrust in pie plate; cover with second prepared piecrust; turn edges under,  and crimp. Brush entire piecrust with beaten egg white, and sprinkle evenly with sanding sugar.

Place pie on a baking sheet lined with aluminum foil. Bake in preheated oven 30 minutes. Cover edges of pie  with aluminum foil. Bake pie until crust  is browned and filling is bubbly, 45 to 50 minutes more, covering entire pie with aluminum foil after 20 to 25 minutes to prevent overbrowning, if necessary. Cool completely on a wire rack, about 3 hours. Top slices with ice cream, if desired.