Cover sweet potatoes with water; boil, then simmer 15 to 18 minutes until tender. Drain thoroughly; steam-dry 3 minutes.
Heat oven to 350°F; grease a 2-quart dish. Roughly mash potatoes with cream cheese, filling butter, milk, filling sugar, salt, and cinnamon, leaving small soft chunks.
Mix pecans, flour, and topping sugar; rub in cold topping butter for crumbs. Spread potatoes in dish; scatter crumbs.
Bake 25 to 30 minutes until filling hot and crumbs begin browning. Add marshmallows in loose patches, leaving plenty of pecans visible.
Bake 5 to 7 minutes more until marshmallows golden. Rest 10 minutes.