Price increases are not possible, quote from the contract "The contract price is guaranteed as an absolute fixed price until the turnkey handover of the house."
Such an absolute guarantee already flirts with the border between bold and brash – you have to be able to afford that, and full pants are easily understandable here. On the one hand, one might not have explicitly anticipated the Ukraine war (and in a globalized economy, a war on the same continent cannot remain without consequences, so in this contractual relationship it will certainly go far beyond the role of a scapegoat), on the other hand, issuing such a guarantee was already reckless and daring: one cannot seriously assume that once "Corona" is "overcome" in the sense of adapted supply chains, a lasting calm must inevitably follow.
The legal assessment of consumer law aspects is therefore only "half the battle." In my opinion, one should also discuss with the lawyer how to deal with the risk of performance at all: I interpret the contractor's behavior as a harbinger of insolvency avoidance and would prudently review the solidity of the completion guarantee discreetly.
even if the house takes longer, that is not damage.
If the contractor only wants to be allowed to take longer without penalty and that is okay with you, then everything is fine.