How to handle joint between uneven ceiling drywall?
1/2" of dip in a relatively small space can be sort of hidden with more mud, but I'd say it's always going to be noticeable.

2 alternatives. First, get some serious leverage underneath (something wide at the ceiling and an angled 2x4 from the floor) and see if you can push the offending drywall up. It may have just been poorly nailed in the past and is sagging.

If that doesn't do the trick, I'd even consider cutting back the offending piece and re-doing it. I get that almost nobody likes drywall work, but since you're doing some anyway, the incremental agony of a bit more will be modest.