Large single-family house with 4 children's rooms - convertible into 2 residential units

Whoever the planner is, he is not worth his money. Best to quickly throw that botch in the trash.

The original poster planned it themselves. There have been much worse things here. I rather see static problems with the large living/dining area. Spans over 5 m with a wall standing on them become expensive. You missed the target of 200 m2 living space.

For sleeping, you can fit all the sheep as you imagine. Four children each get a large room, and if you turn the parents' bed to the short north wall of the house, then they also have a satisfactory sleeping place.
I don't know if the bathroom meets your needs. To me, it feels more like a storage room where you can wash yourself.
".. and around the corner at the back of the hut you’ll find the toilet, in case you need it." I just remember shortly after the reunification when I visited relatives in the middle of nowhere with my mom.

On the ground floor, the grandparents get a sleeping place. The bed might just fit into the niche, but no more. I don’t know how the grandparents want it, but my parents, already somewhat frail in their 80s and my mom relying on crutches and a walker, have a 2-meter bed with separate mattresses, which they need. Also, they need more room around the bed because dad has to help mom into bed from the side, while he needs a short way to the toilet. He doesn’t want to stumble over crutches or a walker. I don’t see such everyday scenes here.

Since I’m already on "scenes," I have to say unfortunately: I do not see scenes of family life here.

overall more efficient floor plan

Maximum efficiency with minimal means. That succeeded: There is no beautiful large and practical patio door through which you can not only walk but also enjoy the view into the garden. Connection garden/interior is what windows in the EFH construction sector are for. There should be space for shaping communal life and communication: corners where toy boxes are stored, where the puzzle is put together, where two or three do crafts while others go in and out because they tinker in the garden. Here everyone feels like they are in each other's way, grandma or grandpa with a walker get knocked over (if running is even allowed in this house, because circling around a bit is a challenge here). I really do not see free development of family life in the living area here. 3.60 as a living and TV corner is borderline narrow.

Room program meets all requirements

Yes! Apart from the requirement that is not mentioned.

Current status: largely final, last optimization round planned

Somewhat alarming!

Actually, I just wanted to write that the wardrobe for 6 people is not enough. At best, one plans about 50-60 cm per person. It may be that every child here can and must store their jackets upstairs in the room, but the more wardrobe at the entrance, the better. And then there is still the footwear. That adds up to quite a lot. There we are again at the necessary space for personal development: the children's and your garden Crocs take up about a good sqm in the lounge in front of the terrace – such a thing is not planned at all.

So I would take the office space as part of the living, dining, and lounge area so that you get somewhat more than just efficient space for living. Also, give grandma and grandpa half a meter more space, plan them more into the common room (parking the walker and watching TV would be a start) and reserve at least 2.50 meters of wardrobe length in the hallway – and that without individual furniture placement on free walls.

But ultimately. If this is your largely final plan, then build it that way.

You will have to massively renovate the upper floor if it is ever to become living space for another family. So in terms of taking down walls, putting up new ones, installing a kitchen, etc. You have probably planned it so that a fully equipped kitchen can also be installed on the upper floor...

If the parents have to leave their own house, they are in need of heavy care; that doesn’t work at all in your house.

Parking spaces will be a complete disaster, with 4 children you are constantly going somewhere. And once your children move in who also have kids... then you quickly have 3, 4, 5 vehicles. Hopefully parking on the street is easy.

You are wasting a lot of space on the upper floor for a home office and guest room. The parents live opposite—why a guest room?

The living room is too small, the dining area for 8+ people as well. Even without visitors, at Christmas you have to sit on top of each other... well...

I now also see the dimension of 2.70 RBM for the grandparents' bedroom. That is a joke. You plan that bigger for young parents. I think the concept of family life was not considered in the planning. I know buildings that have to be built efficiently: office buildings, FUKs, commercial properties. That differs from residential buildings: these should also be designed to be livable. With a good architect, there is then also heart and passion, at least the knowledge of "living."

Quick & dirty and mirrored from functional rooms to living and living spaces for 6-8 people plus grandparent area with own east terrace plus 2 meter wardrobe plus storage for shoes under the stairs

Drainage from the upper floor through the kitchen is something you have to like. The execution has to be right.

If you have renting in mind, then you have to think about the building services separately. Where does the electrical sub-distribution on the upper floor go, where does the network distribution on the upper floor go, ...? How does it look with supply and disposal lines for the future kitchen on the upper floor?

Is public transport so good for you that the parking space issue can be neglected?