Regarding the point: wouldn’t it then be the case that you would even have a better equity/debt ratio, since you actually used more equity in relation to debt than originally planned?
yes, but YOU applied for the loans like that. That means equity first and then loans. In hindsight, no bank will change the contracts in your favor just because you were cheaper.
The bank could also argue that you built an inferior house and they wouldn’t have wanted to finance it in that form. The construction costs are known regionally.