To bring some objectivity back
:
You write that you are engaged. That still costs a lot of money. From experience, 15-20k€ can quickly be gone.
You also said that you can save 2500€ at the moment. Are you already doing that? From my own experience, things always come up. A tip: make an honest budget calculation. Including special expenses or reserves for repairs, etc.
In another post, you mentioned already having 20k equity, but I assume that is still not really there today, right? The problem is that banks want to see some equity; especially to cover the additional costs during construction.
Furthermore, you mentioned that you want to start planning the house already. Do you have a real idea of what such a house construction costs? That would also be a task for you.
By the way, you wrote that you want to plan as much as possible yourself. That is commendable; the architect will still not give you a discount (after all, he has his fee schedule). And with prefabricated houses, it often goes into the area of "custom planning." That means there is an extra charge if you do too much or move load-bearing walls, for example.
And regarding your own work: You want to contribute a large part of the work yourself. There are many other threads about this here in the forum. The fact is: doing your own work costs a lot of time, and time is something that a young aspiring employee doesn’t really have.