Plastering walls / leveling unevenness

Hello,
I am new here and have been reading along for a while. Now I have a question that I haven't found an answer to in the forum yet.
I am in the process of renovating a house (built in 1963).
I have gutted the bathroom.
The old tiles reached about 1.20m high and were applied using the thick-layer method. Wallpaper was on top of the tiles.
Later, I want to tile up to the ceiling, but the thick-layer adhesive projects about 1cm further than the plaster above it. So I need to even out the 1 cm... but how do I do this most effectively?
I think the old plaster is a lime-cement plaster, yellowish-brown in appearance.
Will a new plaster layer about 1cm thick hold there? Or what other options do I have?
Thanks in advance.
Regards, Michael

..... or what options do I have?

Completely remove the old junk! A one-time effort. Everything else is "patchwork".

Best regards.

Exactly, get rid of it - in my opinion, there isn't much left to save and the effort wouldn't be worth it in the end, and you would end up with a rather questionable result at the end of the day.

Exactly, get rid of it

hello, first of all thanks for the answers. the whole thick coat stuff gone...? it is about 4 to 5 cm thick and extremely harder than the wall behind it (bimsstein) I first thought it would be faster to get the tiles down with the thick coat than just the tiles...but little chance. parts of the wall went away because it was/is so hard. regards michael