What maximum sofa width would you recommend?

I wouldn't do it that way, but it has to please you. So the question is how much the opinions here help you. For my own comfort, I would never fill walls that much, and certainly not just fill them with a sofa. I find it nicer when free spaces remain and you also vary in height. Side tables, floor lamps, sofa, pictures, etc. bring variety. But the question is, do you want what an interior designer would recommend – and even there it depends on the living style – or wouldn't you rather do what you like. It will look cramped in any case if there is only 3 cm free on the side. Also keep in mind that only a smaller coffee table fits in the U, unlike in the L. Maybe you don't need it that big anyway, but I would plan it right away. We, for example, like to use ours for lots of snacks, drinks, and decor.

Maybe you want curtains at the windows after all.. That would be impossible with the installation dimensions.

Keep in mind that only a smaller coffee table fits in between in the U shape, unlike the L. Maybe you don't need it that big, but I would plan for it right away. We, for example, like to use ours for many snacks, drinks, and decorations.

We are aware of that there is about 140cm space between the two legs which would be enough for a 60x60cm table

Maybe you want curtains at the windows after all.. That would be impossible with this layout width.

We don’t want curtains

Alternatively, we found 349cm and another with 347cm layout width

I also believe that it will look cramped and uncomfortable.

What do you define as a "gute Wohnlandschaft"?
We are also currently looking and I find the selection overwhelming.

I repeat with green adjustments

Where are the 347-353 cm of the sectional sofas measured? Real or some catalog dimensions, where maybe a backrest is somewhat wider/deeper?

Being squeezed will look modest. In general, you also need storage space on the corner elements – be it for decorations, but also for a drinking glass, phone, or magazine, reading glasses or remote control, nail case, table lamps (!) or snacks.

and would be enough for a 60x60cm table

Not much fits on that. It also won’t be in the "grip zone."

Living room landscapes need space to have an effect and also for the user’s overall spread when using them. Few people only sit or lie down. Even when lounging, there is activity. I would go up to max 3 meters and only in an L-shape, so options remain.