I've got a tricky ceiling transition and some large gaps in my bathroom remodel. The house is a 1960s build with plaster over drywall (and in some areas, additional tile then more drywall). I removed the shower bulkhead. These first 2 pictures are the ceiling area where the bulkhead was. I considered different thicknesses of new drywall to match but couldn't actually get it flush with the existing ceiling in all areas at the same time so this was the best I could do. It's about a 1/4" depth difference in most places from new to old so I think that with plenty of mud, patience, and feathering, I should be able to make it disappear. The metal mesh under the plaster had to be fully removed where the bulkhead was but I was able to cut portions of it out where needed throughout the rest of the corners to save as much of the existing ceiling and wall as possible.

I plan to prefill gaps with 90-minute quickset and possibly use paper tape in the huge crevices like the last picture. Then, tape and mud as usual. Will this work? Are these gaps too large? Are there other tricks or considerations I should be aware of?