Increase living space. Enter the apartment

Hello everyone
I would like to make better use of the room height. Currently, the subfloor including insulation is about 12cm.
I could reduce this to 9cm with flowing screed. That would give me 3cm more room height.
Today, the step into the apartment (i.e. from the stairwell into the apartment including the floor) is about 3cm.

A step of 6cm is probably a bit large and unexpected for guests?
Is there another good idea? I was thinking of an intermediate step of about 3cm in the stairwell.

Unfortunately, barrier-free access is not an option, as there is no elevator in the building.

How high is the room currently, and is it worth the money and effort to gain a whole 3cm?

Today the step into the apartment (i.e., from the stairwell into the apartment including the floor) is about 3 cm.

A step of 6 cm is probably somewhat large and unexpected for guests?

The hallway floor is currently 3 cm higher than the apartment floor?

In that case, it doesn't really matter whether you have a 3 or 6 cm high step down from the hallway. With one step you would still enter the apartment and have another step inside.

Is the hallway currently 3 cm higher than the apartment floor?

Well, if the OP – whose apartments we do not know – "spoke in complete sentences," it would probably say the following:
The apartment floor is probably uneven, and "currently" probably does not mean "at present," but "in the future, according to the current planning status." According to this, the OP intends to install a dry screed construction, which would be 12 cm thick; for a flowing screed only 9 cm would be necessary, thus 3 cm less reduction of the clear height. Currently, one enters the apartment from the stairwell 3 cm higher, possibly this just means a threshold in the doorway. With completely missing pictures, one can see the least.

The floor of the apartment is probably uneven, and "currently" likely does not mean "at the moment," but "in the future, according to the current planning status." According to this, the OP probably wants to carry out a dry screed construction, which would be 12 cm thick; for a flowing screed, only 9 cm would be necessary, so 3 cm less reduction of the clear height. Currently, the entrance from the stairwell into the apartment is 3 cm higher; possibly this only refers to a threshold in the door passage.

Honestly? You use many words that are not present in the original thread. That is too much speculation for me.

Sorry for the confusion. I am trying to be a bit clearer. Attached is a picture of how it is today and how it could be.

and what is the current ceiling height? Is it only about one room and which one?