From condominium to house construction. When do I sell the apartment?

I see it the same way, no one becomes a tenant or landlord. This can be regulated by the handover date.

We were also advised to sell the house quickly and to demand a right of residence for a certain period as a condition. Until moving out/moving to our new house. Either in exchange for rent payment or a reduction of the purchase price. Bridge financing is expensive and not particularly attractive.

With a reasonable apartment in a reasonable location, that's not a problem at all. Many people do it that way (we did too, sold our old house almost 1.5 years before moving out – various neighbors have done the same). You don't necessarily become the landlord as a buyer; that depends on the purchase contract and the regulations. Agreements can also be made regarding the condition of the property. You just have to think about what you want.

Oh, so you received the full purchase price 1.5 years before moving in just like that from the buyer? How did he do that if he is not registered in the land register? And above all: why would someone do that? Do you buy a car and pay today when you might only get it in 1.5 years? If it burns down in the meantime or the market value falls by half due to a new highway nearby, who is liable? :)

As a seller, you are clearly at an advantage. If you find a foolish buyer, that's good, but then it's pure supply and demand.

Oh, so you received the full purchase price from the buyer 1.5 years before moving in just like that?

Sure, that's why the buyer becomes the owner and gets "my" house.

How did he do that if he is not listed in the land register?

When he buys, a purchase contract is concluded, a priority notice of conveyance is entered, and then the change of ownership takes place and voila, the buyer is in the land register – along with his mortgage(s) for his financing.
By the way, this is how it works with every real estate purchase (except the priority notice, which can sometimes be skipped).

Do you buy a car and pay today if you might get it in 1.5 years?

If it's the asked-for Ferrari or something like that! Yes, then I do it. Supply and demand set the rules.

If it burns down in the meantime or the market value drops by half because of a new highway around the corner, who is liable?

In case of fire, the buyer is lucky – if he has good insurance, he gets a brand-new house and not a used shack. And whether a highway is suddenly built right in the middle of the city, I doubt that ;)

As the seller, you are clearly at an advantage. If you find a dumb buyer, that’s good, but then it’s pure supply and demand.

Of course, but that’s quite normal business in reasonable locations. It may look different in the far east, or in the deepest Sauerland or wherever.

For us, it’s "take it or leave it." You can pick your buyers. We found our buyers about 2 hours after our ad was published in Immobilien.... Without negotiations about purchase price, handover date, or such.
If the buyers had tried to negotiate, we would have just said "bye" and taken the next one from the list :)

This also has nothing to do with dumb buyers; it’s simply due to the lack of supply and high demand.