I removed 16" door and bulky framing yielding a 20" opening to linen closet. I can't enlarge opening by myself, so I'm going to frame it and not put a door back on. Problem is the drywall across the top is not flat. It had a visible seam and bump on the upper left corner on which I have used drywall compound to make it less noticeable.
I laid a yardstick across it and found both sides high with a dip in the middle, which I measured: 0.25" in the middle.
Do I keep layering compound until I get it level?
Do I apply the trim and fill in with spackle?

This is a 1976 modular home and this wall is where modules were joined, but not lined up exactly, which is probably the original cause of the problem. The depth of the opening in 8.5".