a very schematic but good drawing: clear and essential.
Given the position of the fixed furniture (in gray) located between the two large windows on the 8m side, the actual perceived environment is not a 5x8 room but, by eye, 5x5. It is still a respectable room, but I take back what I wrote about it being able to accommodate much more than one grand piano.
In any case, I recommend placing the piano in the bottom left corner with the pianist facing away from the window on the 5m side.
In the position you indicated, there are these inconveniences:
1 - playing the piano having a doorway behind you from which people come from other rooms is, in my opinion, unsettling. Playing an instrument is an activity that requires a certain "intimacy" with the instrument, the music, and oneself. Perhaps you live alone, but I must consider all situations (sometimes there are guests, parties, gatherings, movement...)
2 - the proximity of the piano to the fixed furniture: it creates a narrow passage (between the tail of the piano and the side of the furniture)... an impractical solution.
3 - such proximity also creates a narrow view towards the large window and the room in general (with the lid open, it's even worse)... a very unesthetic solution.
The position/orientation that I suggest to you instead offers:
1 - "intimacy" for the pianist, for the reasons mentioned above.
2 - abundant natural light coming from behind and to your right
3 - the lid opens the sound towards the center of the room
4 - while playing, your eyes can wander outside through the large window (if the difficulty of the piece allows it)
Finally, let me tell you one thing: in your room, from what I can see, there are no windows but glass walls; it is quite different and... good for you!
Don't worry about keeping the piano near a glass wall. The rule of keeping it away from a window applies when there is a radiator under the window (not your case), and when the window has glass from past centuries: as thin as a lightbulb and zero thermal insulation (not your case).
In your home in Stans, you will certainly have latest-generation A++ windows from a climatic/acoustic performance standpoint (double-glazed, laminated, argon gas filled, etc..)
And imagine!.. in one of the most advanced building-tech nations in Europe....