Stringers, strength.

[FredrikR]

The fact that there is an attachment point in the second step can make a difference, having two does not make a difference. The post is not that stable.

Haha no, I probably made a bit of a wording mistake, but I meant that the staircase has support at the second step.
I started sketching a saw template for the stringers over the weekend, and it will be closer to 140mm left (perpendicularly, from the underside of the stringer) after cutting. If I then use both a belt and braces and screw-glue, say a 45x120 like that.

The landing feels most troublesome to find a good solution for since a "new" section needs to be added and there are no good support points in one direction, but I am considering plastic spacers, the kind used for window installations, between the landing and the concrete edge that it extends over. https://www.byggahus.se/forum/threads/snickra-vilplan-till-trappa.337955/