I can only speak for how I've handled the chimney in my house. There are certainly others who would have done it differently and many who can offer opinions and point out how much I've done wrong.
The chimney in our house had gypsum plaster, mortar, a fiberboard nailed with wallpaper, and some type of paint on it. There was also a wooden corner strip that went halfway up.
When I tore everything away and cleaned the chimney like yours is, I started with a primer mix. For the corners that looked bad, I attached a corner profile and built up the gap to the list with mortar. I also mortared a brick to make one side a bit more even. After that, I plastered with mortar mix c. On the sides of the chimney, there's gypsum.
I don't really know how you should solve it against the ceiling. I don't quite understand what it looks like from the picture.
I'll try to find some pictures.
Plastering, however, is the hardest thing I've done, and it took me a while to understand how mortar works.