I doubt you can see it, but in the above pic the ceiling arches upward. This will be a bit frustrating while putting up moulding because the ceiling and moulding wouldn’t connect. The internet says ceilings are usually uneven (makes sense if you ever worked with drywall), so what is a good remedy for this?

I know one avenue is to visit my good friend the attic and try from up there (not sure what though), but this is the edge of the house and there isn’t much wiggle room to work in. Another thought I had was to apply spackle as filler, but it’ll be a ~12” x ~12” area to fill up to make it consistent with the rest of the ceiling. I think spackle might be the best route, but only because it seems the easiest. I asking because this is new to me, I’m not sure if my current approach is the best choice. And i couldn’t find any sites that I felt have clear or satisfactory answers.

What does oppo think? Anything I might of missed or another approach that I can consider?

Edit: not sure why I didn’t do this before, but I put the trim up the measure the gap, it’s about 3/4” wide.