"Construction project with a 'special' initial situation"

You already know my personal opinion. However, I believe that you don't really know what you actually want yet.

Regarding option 3, the following should be considered:
- You do not get a pure land financing from all banks (for real estate banks it is not a security eligible for covered bonds)
- Financing amount for the land? Usually 60-80% of the market value (standard land value), the rest from equity
- Bank already receives a land charge = costs
- As long as the financing with fixed interest rate is concluded, no other bank will enter the subordinated position for the house financing. But if you want another bank, costs will arise (prepayment penalty, assignment/deletion/re-registration of land charge). The first bank will dictate poor conditions to you if necessary.

By the way, you can always see cash from the house sale ;)

No, it is also possible if you keep your currently inhabited property (intention to rent) and then mortgage beyond the existing financing. You don’t have to put every euro earned from that into the new property. Otherwise, of course, with any sale – sooner or later. The question is: What do you actually want???