My floor plan for a 4-family house, please share your opinions

??

With the described background (professional with experience, landlord himself, entire family experienced in the industry and local market) you don’t deliver such botched work, but a Holladiewaldfee object (for all I care also with a ventilation bathroom). Admit it: You were joking.

... but without a storage room

Then just leave everything as it is.
If the architect family is so great and we have no idea, then I would build the apartments EXACTLY as they are.

and we have no idea,

... about the local relevance of storage space, we possibly really don't have it

I don’t believe that: as such an entrepreneur, I wouldn’t coach a potential client who just wants me to build a single four-family house. On the contrary: in another business area, he would be my (small) competitor, so he should just build something a bit less attractive than what I build for "myself"

A colleague's parents-in-law did it like that. However, the developer/GC does not act as a landlord or manager—truly only in the construction sector. If he gets the land, he takes the entire profit; otherwise, just the profit margin as GC. In this case, 4 semi-detached houses (according to the GC's standard floor plan) and a really individual and also "usable" single-family house were built. There, the GC's architect proved to be a real architect with his own solutions (but exceptions prove the rule).

Why do you consider the access route catastrophic? How would you do it without redesigning the house?

Because they are extremely convoluted, you constantly have to turn around corners. Without redesigning the house, I don't know - otherwise, I would lead the path straight along the northern property boundary, then bend it southwards on the east side with entrances for the houses on the east side. If the southern house is ever to become the private one, you would have optimal peace this way, and from your garage in the southwest, you could still create a small path through the garden to your kitchen so you don't always have to carry groceries all the way around outside.

But the statement about how likely and how important the eventual private use is seems to be missing somehow.

Our architect is about 60 years old, graduate engineer, sworn expert and has built a few thousand apartments and houses, his wife manages properties, daughter is a real estate agent.

Apart from the floor-to-ceiling windows, the house definitely has the style of the 80s, which fits quite well with the architect's age. Our architect was also similarly old and presented solutions in some places that could be solved differently today...

The daughter will certainly not criticize her father in front of others. I wouldn't either, but in private, yes. If the family also manages and brokers, then with rather difficult-to-rent apartments and frequent tenant changes, you are the better potential customer.