And another floor plan

The floor plan already looks quite good. You surely already have 2 windows in each children's room. Otherwise, the kitchen and the utility room would be too large for me. Instead, I would add one more room downstairs "for old age," but of course, that's a matter of taste... I would also have the garage built flush with the house and put a door in the utility room so that you can go directly into the house from the garage.

Another roof is unfortunately not possible Therefore the HTR has to be quite large, and the kitchen not too small, because all the groceries and such have to go in there as well.

Due to the symmetry and the straight staircase, the kitchen is especially large

Prefer to plan it very differently again?

Flush garage?
Then no window can go there, right?

Do you really want a nail binder roof?

You have no basement and will be glad about any storage space. On a nail binder roof, you can practically only move around crawling.

That is not correct. You can stand in the middle with the binder, just like with any other hipped roof. For us, that is an estimated 2.5m x 3.5m. In total, I have about 5x6 m of usable space under the roof (with smaller house dimensions compared to the OP). Two sides are equipped with half-height Ikea shelves, on the other two sides I have pushed the boxes directly under the roof. The limiting factor is the roof, not the binders.
Only the handling is somewhat hindered by the binder construction.

The roof only needs to be approved for storage.

May I ask why another roof is not possible? For us, the carpenter's roof didn't cost that much extra at all.

That is not correct. You can stand in the middle with a truss, just like with any other hip roof. For us, it's estimated at 2.5m x 3.5m. Overall, I have about 5x6 m of usable space under the roof (with smaller house dimensions compared to the OP). Two sides are equipped with half-height IKEA shelves, on the other two sides I pushed the boxes directly under the roof. The limiting factor is the roof, not the trusses. Only the handling is somewhat complicated by the truss construction.

The roof only needs to be approved for storage.

Then it probably depends on the construction. I've seen truss roofs where there were braces everywhere.