but I have never had to take the board from under a wall, which
I will have to do in the closet.

I would just do usual leave 0.5" - 1" on the inside, add new 2x4
support,
glue & screw new subfloor and add 1/4 plywood to cover entire room.

In many mobile homes subfloor glued, screwed and nailed to the point
where you have to remove walls.