Land available but only a condominium?

Yes, and it might only help to start building when the second child is here and both are working again. The advantage of this is that the entire income and expense situation is fixed. Currently, I can’t imagine how you can build such a house with reduced working hours. I had assumed that the 2300 are already part-time. Unfortunately, there is not only a threat of income reduction due to part-time work but also additional daycare costs:

in anbetracht der kita kosten willkommen wieder im süden = schnell mal 450euro pro monat.

Phew. When I read about the (clearly) above-average salary, I honestly thought of 6-7,000 net, for both in full-time.

5,300 EUR is good, but for BaWü and your plans, it's already quite little again. Well, then you get child benefits, but that at best compensates for part-time/parental leave.

Especially since it’s full-time, and from my own experience, I can hardly imagine how you want to manage 1-2 kids and a house with a lot, a lot of work in the first years with 2 full-time jobs.

Leave out the basement and garage, does anything else really make sense here?

Yes, and it might only help to start building once the second child is there and both are working again. (...)

From my own experience... no, because:

With 2 (small) children, both parents can never work full-time, no way.
With 2 children, you hardly have the nerve left for the many, many, really many clarifications for the new build...

And... before both are working full-time again with 2 children, at least 5 years will pass from the birth of the youngest child... if it can't be done differently, at least 3. No way, in whatever world, can my wife with 2 children (6 and 1) go back to working 40h/week in a few months...

Yes, time will tell. For some, the parents live nearby and have time, and somehow it still works out. Whether that is the path for the OP is questionable, most likely not. They are still at the very beginning of the journey. Apart from a few (unrealistic) wishes, there is no kind of plan. Not even the daycare costs seem to be known, although those could be quickly found out at the construction site.

what is your conclusion from the thread, how should it proceed, what are your next steps?

We have actually built twice before or during parental leave. I always found it quite good for that reason, because I knew "in the worst case we're basically done, it can only get more relaxed."
With the first house, the child was small and I was still on parental leave (with half parental allowance, I shared the money for a few months). We always knew that the first 1-2 years we would have to tighten our belts a bit, but we keep a casual household budget in Excel and knew that we would manage with the loan installment and our expenses. Vacations and savings rates were somewhat lower during that time.
After the parental leave, there were relatively quick job changes with good salary increases, so if we hadn’t sold the house, the burden from the loan would have really been manageable afterwards.

With the second house it’s similar now. We are about to welcome our second child into the world, and just a few months later the repayment for our new house begins. Again, we are at our "lowest point" income-wise, but we have calculated it again using Excel and know that it will work. Just the "living large" is missing for a year or two. But I know that as soon as child no. 2 can go to kindergarten, it will be much more relaxed financially again.