I haven’t read everything.
120k EL is unrealistic. That’s labor time and assuming 50€/h, it would be 2400 hours of work. So 300 days of 8 hours work.
You basically have to build the house almost alone because the remaining 280k is material and that’s hard to save on.
If you really want to do some of the work yourself, find a small company to do the shell construction. Maybe you can work there as a stone carrier, but you won’t save much more than that. The whole thing is also a warranty issue for the bank.
You can manage the finishing work with connections alone. If there are no extravagant things in the electrical or sanitary installations, that’s even doable. The only thing I would have done with a company is the final inspection. You have to do that with electricity anyway.
Regarding money, it depends entirely on the bank. With the online bank, you will probably have to prove every € somehow. We stayed with the house bank and declared far more than the average in EL. In the end, there was still 50k credit available for it. Both full-time jobs and never had a loan or property before. They didn’t want to see anything and just transferred everything to the checking account shortly before the interest-free period ended after a phone call.
No one wanted to see receipts afterward. Only photos of the current state were requested monthly.
Chasing 2-3 weeks is probably only needed if two left hands are at work and the house is 200sqm+. Electric work in EL without prior construction knowledge. I never held a router and we went through the whole house in one weekend. And we certainly didn’t skimp on this trade. Even wiring is a joke nowadays. The only problem might be the control cabinet.
Sanitary/heating same topic... Once the shell is up, you can lay the few pipes yourself. Insulation on and a trained installer takes care of the last connections. You just have to find the right companies, but there are some who gladly take on smaller filler jobs without pressure on deadlines.
Anyway. As a layman who really has to keep an eye on costs, the way to have the shell built and the rest done/awarded in EL is one of the best methods. Still difficult without connections. And say goodbye to planning with acquaintances... You can involve them in the end, but at the beginning there are simply too many appointments that have to be kept and when people constantly drop out, even more problems come up for you.