Wooden ceiling and partial insulation for basement with garage

the naked path bath to the bedroom or KiZ would also motivate me to mirrored glass panes.[Nachbarn?] The sauna is preferably 2x2m interior dimension, so that 2 people can lie diagonally next to each other at the top. There are those modern glass sauna/shower solutions

The garage is of course separated from the basement by a wall with a door. We would also insulate this. However, the topic of fire protection is certainly an interesting aspect. Is that compatible with the wooden ceiling?

The large windows upstairs are primarily meant to bring light into the house that we lose due to the large balcony on the ground floor (which also has advantages in midsummer). In winter, they also generate solar gains. The entire upper floor should be shaded with S-Onro roller shutters so that it doesn’t get 40 degrees in summer. Nevertheless, the roller shutter can be adjusted so that enough light still comes in. Unlike blinds, they also work during wind. Privacy from the street should later mainly be provided by a hedge and a large tree. Until then, the roller shutter will probably be down more often. The remaining budget then determines the size of the tree.
I find mirrored glass quickly looks like an office building and is useless in the dark.

The garage is of course separated from the basement by a wall with a door. We would also insulate this. The topic of fire protection is certainly an interesting aspect. Is that compatible with the wooden ceiling?

As far as I know, the garage must be separated from the basement rooms by a fire protection door (at least T30). These doors are also available reasonably well insulated, although in a basement this probably plays only a subordinate role. Nobody has been bothered that we store wood in our garages. However, it is also stored in the garage. A wooden ceiling would have contact with the living space above. In my opinion, that would make a fire protection door absurd. But wood can also be equipped to be fire resistant. There are certainly regulations and ordinances on that :)

A bathroom with only 4.4 sqm in the granny flat is possibly quite small for a person in need of care. [...] I consider (!) a children's room with less than 12 sqm to be unreasonable

You probably mixed up the terms. A children's room with "only" eleven or ten square meters is quite small, and a care bathroom below eight square meters is already unreasonable.

Plan B would be to close off the gallery, although that would really hurt us since it is literally a "highlight" of the house.

This house will not suddenly become ordinary just because you leave out the gallery. It doesn't always have to be caviar; gags and gimmicks also wear out in everyday life.

mirrored windows (neighbor's hunting stand)

Discretion depends—assuming the appropriate upbringing—on the viewer. Learning this, one may reasonably expect a certain degree of personal responsibility from the neighbor.