Floor plan design single-family house with around 150 m²

Not in addition to the sofa, that would make it way too crowded again.

I think it's good if not all guests sit "in a row," as it would be with the current proposal. That's why I originally had the idea of the corner sofa, so 2 can sit on the south side and 2 on the west side.
With an armchair, at least one more could be accommodated. Or maybe just 2 separate sofas – a 2.5-seater on the south side and a 2-seater on the west side? Probably too crowded again?

Hmm - even if you sing the song of cluttered living space to the living space - doesn't that make it a bit sad?

I would find it a shame to leave out the piano now - because of it, I have already given up windows to be able to fit it in at all, which could have been placed in its place. I would be really annoyed about that now.

I think it's good if not all guests sit "in a row,"

That's why I believe individual armchairs - complemented by carefully dosed multi-seaters - are the best enemies of the chicken-bar seating arrangement.

That's why I think individual armchairs - complemented by finely dosed multi-seaters - are the best enemies of the chicken-bar seating arrangement.

A seating option on the west side would be important to me, if only to be able to look out the window on the south side once in a while.

No, exactly that is the difference between furnishing and cluttering or just shoving it into the corner. And there actually is no problem with the passage there. Besides, you can sit in the middle and watch the fireplace blaze. Or turned, look into the garden. Maybe a simple long sofa with a separate cube/footrest, then you are more flexible.

Well, I also don’t think that the problem shown at the beginning is a problem, but for the OP it clearly is, and then this section might(!) become a problem again because you would have to zigzag to get from the back part of the sofa to the fridge (the problem with the table was described in a similar way). Hence my (and yes, also your) advice to use a flexible sofa.

Something very basic... would it be worth thinking now or sometime in the future about additional windows or window widenings etc., could that give the room even more? I find it already very wall-heavy.
Or can the room also be made to feel large enough with design means so that you don’t feel too cramped?

Do you really feel cramped or is it just uncomfortable?
Everyone here has confirmed it to you, it’s unfinished.

Search at ploetzlichbauherr (it’s a building blog) for living room painting, they have a living room with even fewer windows, but it doesn’t look uncomfortable now. It’s not my style, but you can feel comfortable there nonetheless.

I would find it a shame to leave out the piano now – I would be really annoyed about that.

But you are annoyed anyway. If you block a potentially beautiful and cozy room with the piano, that’s even worse!

Well, I also don’t think that the problem initially pointed out is actually a problem, but for the OP it is, and then this section might possibly(!) become a problem again, because you would have to zigzag to get from the back sofa part to the fridge (the problem with the table was described similarly). Hence my (and yes, also your) advice to place a flexible sofa.

Flexible is great, the idea of the attachable/detachable ottoman is brilliant, you can place it anywhere as needed. I think at that point the section/ottoman in the room wouldn’t bother me, the walking path is probably along the TV/fireplace towards the dining area/kitchen.

Do you really feel cramped or is it just uncomfortable? Everyone here has confirmed it to you, it’s unfinished. Search for “living room painting” at ploetzlichbauherr (it’s a construction blog), they have a living room with even fewer windows, but it doesn’t look uncomfortable now. It’s not my style, but you can still feel comfortable there. But you’re annoyed that way too. If you “block” a potentially nice and cozy room with the piano, that’s even worse!

When I look from the living room door facing west, I don’t see a balance between wall and window proportions. The center with the large sliding door looks great and inviting; to the west it looks rather like a dark hole (slightly exaggerated). Especially on the west side, the small window almost looks ridiculously small and lost. Also, the south window, for example, is narrower than the wall beside it on the left (piano wall), which looks totally odd and also lost. That’s why I asked if this can be saved at all through furnishing, since the wrong proportions still remain. Of course, you can also see the whole as a cozy “retreat” where you are hardly disturbed by external influences, but when I see other current modern houses, they invite me more. It doesn’t have to be a glass palace, but a bit more balance, well-dosed, would have been good. I even know the construction blog, and their living area is indeed cozy, even though I find it a bit too closed off as well.