Payment plan is somewhat okay or clearly optimizable

Then it is not a property developer but a general contractor who arranges a plot of land for you (which is unfavorable for the real estate transfer tax, but that is another matter). Thus, you pay 63 percent for the closed shell construction. I personally find that quite (too) much. (The benchmark is about 50 %.)

Compared to all GUs whose payment plans I have seen, I find this one relatively good. 50% for the closed shell including windows I consider to be unrealistic. It may theoretically work, but I have never seen it before.

Then it is not a developer but a general contractor who mediates a plot of land to you (which is unfavorable for the property transfer tax, but that is another matter). Thus, you pay 63 percent for the closed shell construction. I personally find that quite (too) much. (A benchmark is about 50 %.)

I already found the Gartenschau plot before. But I only get it if I build with Massiv Haus Sachsen or Kern-Haus... no idea what is going on behind the scenes. My actual favorite was the Bauprofis from Leipzig... but that does me no good either if I simply don’t get the plot.

That may be all true. But the property here is not from Massiv Haus Sachsen GmbH.

That's exactly why I asked about the developer, which you explicitly confirmed, but that is incorrect. Then the payment plan is rather unfavorable for you. A bit less until the shell construction (including windows) would be better.

Thank you for the answers. Is there even the possibility to renegotiate something like this here?

This is what the standard looks like according to the Broker and Property Developer Ordinance; with your agreement, you are clearly better off (for whatever reason your developer puts themselves in a worse position – I don’t even know if they are legally allowed to do that):

That may all be true. However, the plot of land does not come from Massiv Haus Sachsen GmbH. But I can only get the plot through them.

The one from Nordanney includes the land, YOUR payment plan does not include the land.
I don’t find it attractive, but most payment plans are like that. You pay over 50% for a shell construction. Still, you hardly have any opportunities to influence a contract part that was worked out long beforehand with a notary, etc.