New standards in new construction and their impact on us

You would have to tackle that, however you want to go about it. That can be discussed.

Actually quite simple. Housing companies are obligated to agree to an exchange of differently sized apartments without increasing the rent.
No loss for them, little effort, great benefit for everyone.

(...)
See my attitude towards the garden. With an apartment, I would immediately weigh small rooms against a “huge” roof terrace in favor of the outdoor area.

Yes. Because you can afford it. In my birth city, we are now at 17-20 EUR/m² for new buildings, 12-15 EUR/m² for old buildings in good locations. So just the 20m² balcony quickly costs 200 EUR cold rent.
The average person can no longer afford that, so everyone fights over the old buildings, hence the shortage.

Rumors have long been circulating that in many of these apartments no one actually lives permanently, but that they are bought as investment properties or for AirBnB or simply by people who are in Hanover 5 or 6 weeks a year and do not want to stay in a hotel during that time.
That will not apply to the majority of apartments, but to some.

Actually quite simple. Housing companies are obliged to agree to an exchange of apartments of different sizes without increasing the rent. No loss for them, little effort, huge gain for everyone.

Haven't I read somewhere that the majority of rental apartments are provided by private landlords or small inheritance communities or GbRs, rather than the big ones? How do you handle it there? And then you also have to convince the old people. Even 3 streets away is already a world trip for someone who has spent 50 years of their life in that one apartment. That's why, in theory it's simple, but in practice incredibly complicated.

My conclusion:
Build smaller, do without frills, lower expectations... or do it yourself...
When I think about it. No one here has parquet flooring in the places I’ve visited so far. I only installed click parquet myself in the living room, and that was already ridiculously expensive. Everywhere else is laminate... with small children, that’s definitely the best, the parquet has suffered a lot, what a shame.

Some people here live in a strange world where all of this is supposed to be absolute minimum standards, which I always dismiss as frills.

Nobody here has parquet that I've visited so far. I only installed click parquet myself in the living room, and that was already ridiculously expensive enough. Everywhere else it's laminate...

Although cheap parquet is not more expensive than vinyl or expensive laminate. Laminate is actually just plastic – I really wouldn't let that into the house (looks and feels dumb).

I once read that the majority of rental apartments are owned by private landlords or small inheritance communities or GbRs, rather than the big ones? How do you handle it there? And then you also have to convince the older people. Even moving just three streets away is already like a world trip for someone who has spent 50 years of their life in that one apartment. That's why, in theory, it is simple, but in practice incredibly complicated.

I don't know what the share of companies and small landlords is. But you could start with the companies. And some of the older people would certainly move. Not at 80, but maybe at 60 or 70. Because by then, the kids are out and the apartment is too big. You could simply start with that and then see what happens. And if "only" a few hundred thousand large apartments become available that way, it would help the cause.