"Another possibility would be that I service the loan alone. With the installment we have in mind (2500 euros), 1700 euros would remain from my salary. Would the banks approve that? I think I read that more than 35% of the salary is not accepted."
The banks have standardized living expenses stored in their systems for creditworthiness assessment. This means that depending on the size of your household (children), an estimate is made of the average living costs and whether the capital service for the loan can be serviced from the remaining amount. These calculations are provided to the banks by the respective interest groups, so they can differ between savings banks, cooperative/banks of the people, public banks, etc.
PS: According to current statistics, however, over 40% of the household net income is spent on housing in metropolitan areas. Of course, 40% of 1,000 EUR net is a different matter than 40% of 10,000 EUR net. With the latter income, there is still something left for an ice cream and a pizza at the end of the month.