A construction tip - plastering a wall made of cellular concrete blocks

I am looking for advice. I built a dividing wall in the apartment using concrete blocks. Then I tried to plaster it with the Atlas plaster mix 6-30 mm. Unfortunately, lacking the proper skills, it turned out uneven, I did not level it out and did not smooth it out. In order to lay another layer of plaster to even it out, there is not much space for thickness. It seems to me that I would have to sand down the unevenness and lay another layer, but with plaster that allows for a thickness of 2-3 mm. Adding another 6 mm may be too much :( Can you suggest something, advise? What kind of mortar do you recommend, with smaller grain size, that will allow me to level out this plaster and then apply a gypsum finish on it? Please have a kind approach to a beginner who is trying to do something on their own.