Level and plaster the ceiling in the bathroom

Hello dear DIY community,

some time ago I had my bathroom renovated, then noticed that the ceiling is uneven and now first needs to be leveled, then plastered. You can see how it looks in the drawing. The ceiling is a concrete slab in a high-rise building (built in 1972), this concrete slab is now warped, see drawing. On the right it is still even, from about 60-70 cm the warp begins, on the left there is already about 35 mm deviation when you place the spirit level.

Now I want to level the ceiling and then plaster it with plaster for damp rooms. As I said, I want to first level the ceiling from the left towards the right side and then plaster it. My questions about this:

How would you approach this? I thought about leveling the larger gap on the left with a 12.5 mm drywall board and leveling what is missing with filler and plaster.

What plaster thickness, material thickness is max allowable or recommended for ceiling work? I am also considering just leveling the whole thing with material without the drywall board, would you recommend that?

Which materials should be used here?

Thank you very much for your answers.

Regards Orban

Hello esteemed DIY community,

It's actually not here - this is a house construction forum. But I have a suggestion: Leave it as it is! Everything you do to it won't look better. It would already have to be patched. The ceiling looks quite good overall. I would put a finishing strip after painting and that's that.