I also think that you should just do it now.
With a calculatory 6% capital service per month, you pay €500 per month per €100,000 loan to the bank. With a (quite low) target rate of €2,500, that is a €500k loan, plus your equity. So you actually have more money available. During the construction phase, you will continue to be able to save equity, which can also be used for subsequent things like the kitchen and outdoor facilities.
Would you like to explain to us why you want to wait until 2025? What do you hope to gain from waiting? You will also continue to pay rent, and construction costs will probably not decrease, at most stagnate.
And generally: Why can’t you set aside more money monthly?