Evaluation of construction project with maximum own efforts

What do you think the material costs?

You need about 20 m³ of concrete. The concrete mixer delivers it for 2000, at most 3000 euros. Reinforcing steel. Maybe another 2000 euros.
Formwork boards, planks, styrofoam, screws, battens, KG pipes, 2000 euros. 500 euros for your 2 helpers. That's it.

4 weekends of work for yourself. With a company it easily costs you 25,000. With self-construction only 8k.

Please check the position Bodenplatte in the offer from a GU and compare prices.

One might perhaps calculate like this: 1 skilled worker hour = €55 (or whatever is paid in your area).
And then, for example, interior plastering: 2 men, 4 days = €3,520 savings.
Fixing the roof truss: 5 men, 2 days = €4,400.
Electricity: 1 man, 10 days = €4,400.
Screed: 3 men, 2 days = €2,640.
And so on.
If you want to come out with, for example, €150,000 savings, 15 people would have to sacrifice their annual vacation for your construction.
Further savings potential I see in buying and subsequently selling machines that are used frequently ([Radlader], [Rüttelplatte], [Betonmischer], [Gerüst], large trailer, etc.) – often you get back the purchase price if you buy used beforehand and save on rental costs.
And, as already mentioned, if wage labor is done, then cheaply by helpers or unskilled workers who are guided by one of your friends.

Since Edit doesn't work, a small addendum:

Also, of course, buy the materials yourself. An entrepreneur has to add a surcharge of around 10% if they procure materials and sell them on to you (at least that's what I was told). You could go to a building materials dealer you trust, set up a builder's account, negotiate a 10% discount, and always order large quantities so that you move down the price list.

If I stick with KS blocks today, 2 men and a hand saw are completely sufficient. The bricks are bonded with thin-bed mortar or sometimes glued with foam nowadays. It's like Lego for adults. You can't work easier, cleaner, or faster.

And building yourself is also a lot of fun.

4 weekends of work for yourself. With a company it easily costs you 25,000. With self-build only 8k.

The OP, the questioner, doesn't want to bricklay himself but rather rely on the craftsman family. He himself can do electrical work.

This 100k difference in own contributions is not insignificant.

The different figures only come about because everyone bases it on their own region and experience. These are all rough back-of-the-envelope numbers. One knows the cheap north, the other the expensive south. I try to average out the north-south gradient. We don't even know if you want to live or build in an expensive or cheap region. Don't make the mistake of only reading what you want. That is driving straight into the wall intentionally.

I'm already looking forward to the grill and beer when Bausparfuchs wants to shape the KS with the Fuchsschwanz. xD

I have drilled and milled ElektroSchlitze into the KS 2.0 solid brick at my place, and I would say, get a patent for the Fuchsschwanz if he manages one stone.

As such, I would always prefer KS + WDVS over aerated concrete (garage made of aerated concrete).
Back to the topic.
For example, we did the excavation up to the underside of the foundation slab ourselves, compacted 400t RC material,
costs 6k€ vs. 20k€, so you roughly have a guideline value for this position (without excavation, slope, etc.)

I already find it very positive that you have many trades in the family. Do they all have the time and desire to work for you for free? Regarding the material costs, they should also be your first point of contact.

- keine ausgefallenen Erker, Vorsrünge oder ähnliches

Very unfortunate, especially since you have bricklayers and roofers in the family.

mittlere - gehobene Ausstattung

What exactly is supposed to be upgraded?

Maybe a good approach would be to first find an architect and plan and design the house. Then you can break down the trades, write down prices, and discuss with your dear relatives for how much money or material costs they can and want to take over.