Payment plan is somewhat okay or clearly optimizable

I could get endlessly upset again about why there is actually a tax difference depending on whether I have the house built by another company or by the one that sells me the land. But unfortunately, getting upset doesn't help...

Don't get upset: buying and building are two completely different things. Buying/building... that's how it is explained. In one case you are the buyer, in the other the builder.

The company will not sell me a plot of land again.

And how does the property seller want to force you to use a specific builder/general contractor then?

I came across the property via Eilenburg itself (municipal administration).

Here I had myself put on the reservation list (through the project developer). These are still being developed.

Then I was offered a plot of land (30 are being created) by Massa Haus and Kern-Haus, of course with the intention of persuading me to buy a house.

Afterwards, I contacted the land developer. He found it scandalous and thanked me. With the note that I will definitely get a plot.

A month later, I now found out that Massiv Haus Sachsen is also advertising with these plots. Here I was told that only Kern-Haus and these ones are allowed to build there. That is what the project developer wants.

So I talked to him again on the phone; he was evasive but eventually implied that actually only Kern-Haus and Massiv Haus are supposed to build. However, I shouldn’t worry and should continue planning with the Bauprofis.

In the meantime, I had contact with Massiv Haus and am now where I am.

The boss of the Bauprofis can’t help me here either. He told me they certainly have a deal with some commissions... of course I end up paying that, probably included in the house price.

Once again, the company is not selling me a plot of land

So you can just buy the plot of land like that and then calmly reconsider who you want to build with? Then just look for another builder with a better payment plan.

But if you don't want to believe the claims here that combined transactions of land purchase and house construction are jointly subject to real estate transfer tax, then you should simply be able to buy the land now, especially if it comes from the municipality.