Savings potential in turnkey prefabricated houses - assessment?

That you live rent-free already takes away the cost factor.

I can only compare what we have experienced and what is happening in our circle of acquaintances/village.
1 house was almost entirely done by own labor, was finished very quickly. However, almost every trade is represented in the family, and hardly any companies were necessary.
2 houses are almost entirely contracted out + normal own labor.
Construction started in 2017 - end still open, one doesn’t even have windows yet, the other is now being plastered.
Both also complain about the partly high offers.
The costs are getting out of control (although it also depends on the planning basis - therefore subject to reservation).

For us - everything planned through the general contractor went well and quickly, the rest was discouraging. In October 2017 the contracted company said they would not do the garage roof, no time. In January 2018, through contacts, we finally found another company - which started this morning. Agreed was May 2018. Just as an example.

The V1 is 134 sqm in size, I come to 1950€/sqm, which is already built very nobly.

That also includes the Kfw 40+ "package" which costs several tens of thousands of euros. However, we don't want to build KFW 40+, so I think a lot could be saved there and then possibly invested in more features.

You cannot simply deduct this with some tens of thousands of euros as a flat rate. Viebrockhaus builds nothing else. Their entire house concept is designed accordingly, and they build in volume.

Take the floor plan and request a quote from a smaller regional general contractor who builds houses according to the Energy Saving Ordinance and can achieve KFW 55, if necessary.

What do you want to do with EL?
It doesn't make sense to look at Viebrockhaus to see how much they offer a house for and then subtract amount X and plan with that.

You want to do a lot yourselves and build freely, so you need an architect. Find one and plan the house you want to build.
Have the architect then give you a cost estimate for this house. Every trade is itemized there.
Then you can verify this cost breakdown by getting quotes from the trades you won’t do yourselves.
For trades that you do completely or partly in EL, you can check what the materials cost and then subtract the labor from your cost planning. For some trades, the labor costs are only about 30%, for others, labor accounts for up to 60%...

But as a note: Viebrockhaus does not have to create their own structural engineering or building plans/permits, etc. for this house. They build this standard house in large quantities. You or your architect must create your own plans and have the structural engineering calculated (done), etc.
Furthermore, Viebrockhaus buys the materials at completely different prices or even uses completely different materials (a house is made of more than just bricks)
Therefore, I don’t find your approach to be purposeful.

Definitely there will be more costs if you plan and build it yourself. The only advantage would be the individuality. Deducting and calculating nicely from the price will get you into deep trouble with the budget planning.