I can already understand the problem. Top salary at 31, everything done right job-wise. Unfortunately, lived comparatively with comfort (2k€ rent) (assumption), which is why so little equity is available now. If you save another 2000€ per month for 5 years, it would look better...
I would gamble and negotiate the seller down further. Since you’ve been negotiating there for so long, he doesn’t have any other candidates?
For comparison: Our house went down from 850k€ to 720k€ (two-family house) also with a large plot. In hindsight, I learned they only had offers for 700k€.
I had estimated the renovation costs lower. Architect says with 60-70k€ you don’t get that far (room layout, new kitchen). Bigger bathroom will be done later. Roof will cost 100k€ in 10 years with external insulation. The money also has to be saved first. This is possible for us as the current installment is 2.5k€ (700 rent income) and DINK with 7300€ net. With children, it would look like you, IGM AT with 120k€ gross (which I assume you also have).
As I said, I would push hard in the negotiation. I talked to the building society financier this morning; he said normally he gets 3 financing requests per day, currently 3 per week, of which at most 1 is still financially viable! Nobody is able to buy houses at these prices anymore!
Good luck!