Our house is also exactly 8 meters wide and we also have the living-dining areas side by side with the kitchen behind, but we don’t have a TV-fireplace partition wall. In terms of spatial feeling, I can say that 8 meters would be sufficient for such a wall. Cable routing in the floor then has to be planned in time. You could possibly also place a bench against the partition wall, which saves space at the dining table. I find it very nice anyway that someone here doesn’t want to place their sofa in front of the window, but would rather look towards the garden.
Children in the north and the parents in the south facing the garden I find rather suboptimal; here I would swap the sides. The dressing room would then have to be different. Although I already like the upstairs floor plan as it is! However, the balcony would then also be difficult; maybe an exit onto the garage would be enough? Some people actually hang their laundry there. Although I would also switch to a carport or parking space and rather create a closed extension or a garden shed for storage options. But keep in mind that you cannot store everything in unheated rooms. So you also need storage space in the house, and that is currently sparse. Do you have an attic?
Regarding the guest room downstairs: You yourself don’t want the stairs in the dirt area, but your parents are supposed to live there and walk barefoot through that area to the bathroom. It is very commendable that you want to create an option here for later, but I doubt that two people would be happy in that tiny room and the tiny bathroom right next to the front door, especially if they require care. Who says that only one person will “remain”? As an office and guest room, I still find it an important room. However, I would probably eliminate the shower there—for the above reasons—and keep the children's bathroom upstairs. Otherwise, they would also walk through the dirt area. Once the children have left the house, a stairlift along with your parents could then move in.