I’m curious the best way to blend to different profiles together. The hallway floor is engineered hardwood T&G 1/2 thick and we are wanting to remove carpet and match. A 99% close match is 9/16 thick. Suggestions would be appreciated.

Use an edger with 16 grit sandpaper to take some of the sub-floor off. Start at the doorway with the first plank so you know if the heights match. Then taper the grinding off so the sub-floor is flat again.
Or plane the bottom of the first plank so the height matches with less planing on the leading edge. An electric plane will make it easy, using a board that goes across the door in one piece.
Don't try to match the T&G. Glue the first board down.