Direction of laying parquet / parallel or perpendicular to the hallway?

The argument about cutting lengthwise is valid. On the other hand... the 6m floorboards are far too valuable to saw them into three pieces.

There is still enough space for it in the allraum.
Between 2 and 6 he wrote

Thanks in advance for your answers. And I am the parquet installer myself, and I'm an amateur. At the beginning or in difficult spots, the professional still helps me, the rest I do alone or with other amateur buddies ;) Unfortunately, I don't know how many 6m planks there are, actually it was said that there are planks between 2 and 5 meters, according to the offer, but he now said there are also some 6m ones. The hallway directly after the front door is 2.5m wide, from the stairs it briefly narrows to 1.5m, but it is an open staircase with a glass railing, so it shouldn't feel too cramped (currently still a wooden railing as a provisional solution, hard to judge). The same on the upper floor, 2.5m wide and along the railing only 1.5m wide. Basically, the hallway upstairs is 8.5m long, so the long planks would work well there. Upstairs in the hallway, we don't have an attic (roof equals ceiling), so there's plenty of headroom. I'm really struggling with this, I feel like I would have laid it parallel to the hallway direction, it's rather positive if a hallway looks long. But most opinions on this, including here in the forum, tend to favor laying it crosswise in the hallway. Regards

It doesn’t just make it look long, but also narrow. You then get a tunnel effect. Trust me, across is better. Especially for a layman when installing.

Professional from YT means perpendicular to the entrance or lengthwise to the room.

Interestingly, we have almost exactly the same floor plan on the upper floor as you, and I laid the floorboards lengthwise. I like it, and so do everyone else who has seen it so far. Do you have a chamfer on the floorboards? That was one reason for me to lay the boards parallel to the light incidence (and thus lengthwise).

You can also manage the longitudinal cuts. You just have to not be afraid and take your time.
It's also important to measure the width exactly beforehand and possibly start with a half board. Otherwise, it will be awkward if 2 cm remain on the other side of the room.

Have fun!