Is the floor plan for the living room and hallway too narrow?

The site plan, north arrow, information on knee wall height, longitudinal dimensions, etc. are missing. Everything that I would write about the rooms has already been written.

It should also be mentioned that the living room in the front area does not measure 3.88 RBM, but rather yields a traffic path around the door width. You cannot use the full width there.

Ground floor wardrobe, left:
the 61.5 cm in the shell construction (!) should rather be 65 cm to later obtain a practically usable 60 cm.
The rest has already been said.

Some things just don't fit well together. Here: house size and centrally located straight staircase.
Your design makes living cramped.

If it has to be the straight staircase, then you could achieve spaciousness at the cost of increased noise transmission, lower hallway comfort, higher risk of accidents, and higher costs:

Build the staircase narrower and steeper than a steel folded-tread staircase that filigree extends into a large room where cooking, eating, and lounging takes place. Instead of the "box" that divides the essentially large room into three unfavorable parts, you now have a visually light eye-catcher that restores spaciousness while structuring the room. In this case, I would also skip the two glass doors and leave the room open to the hallway. It should be considered whether the first three steps are made as a wooden platform with hidden storage.

Upstairs you can arrange the bedroom and bathroom over the living area and move the children’s rooms to the entrance side. The roof shape should allow the children’s rooms to be equipped with a sleeping mezzanine, creating much space. With some sacrifice of lavish space in the bathroom and bedroom, a narrow gallery-like light shaft can even be integrated usefully.

However, such a design can no longer be built by a "normal" developer or prefabricated house manufacturer...

After a long and exhausting planning process, I now feel that we were a bit too frugal with the width of the living room at 3.88m raw measurement.

First of all: You are in the best company here with what feels like a hundred aspiring builders, whose first threads start with being not quite sure about a single tiny point of genius in their painfully born design – and who then suffer being told: uhm, you have to be very strong now, go back to the beginning once more.

I had such a living room for a long time, and it was never too narrow for me, not even with a piano. The problem of "your" design is quite different, and the effort of the planning is connected with the torment you are imposing on yourself. There is blood in the shoe, the right bride is still sitting at home. Better build a house for your family.

What you instead tried to do is: squeeze a Jette onto a Flair plot. That it hurts is not a bug but a feature: it is supposed to warn you to turn back.

In this respect, I do not understand how my predecessors can even give you tips on individual parameters, when even a blind person should be able to see from the moon where the structural problem lies: You are trying to copy a Jette – stop it, or find the right plot for it. A Jette in a light version for those with statutory insurance does not work, it can never be anything but a struggle!

Heinz von Heiden has a better solution for a staircase in the hallway upstairs in the floor plan of the Arcus 180 ... if it has to be a staircase like that, take a look at it ...