It is basically true that a price is a price (by the way, fixed price is just a coined term, not a legal definition). But that doesn't help you if the general contractor then leaves you out in the rain (whether in terms of time or quality), or in the worst case goes bankrupt because all his customers are unwilling to make concessions.
Therefore, you have to deal pragmatically with the demands of the GC and not just react with "But that's a fixed price".
No, it is not. The contract is legally binding. The GC MUST build you the house at this price. If bankruptcy occurs, then we are dealing with a completely different case – no question. In addition, he MUST also build my house; that is what the contract was concluded for. Only bankruptcy releases him from this contract. You have to be much "tougher" as the client. Ultimately, at the current stage, almost every GC will try to squeeze the maximum out of the client.
I don't even want to know how many GCs take advantage of this situation and try to squeeze out the last penny... See Corona aid – exactly the same game. If you have any opportunity, you try to increase the profit.
Entrepreneurial risk... That's just how it is, but for that, he also earns a lot of money. If he can't manage that and doesn't manage to add a markup on the next houses, then he is also partly to blame. The order books are full!! They definitely will still be full in two years. Then you just break even with 2-3 houses...
People are currently even willing to pay 30% more? That won't end anytime soon...
Ultimately, no one will thank you for it... You have to defend yourself. Personally, I wouldn't even be casually willing to pay 5k more for the same service just because prices are rising right now? What do you think how often prices have already dropped by 10% and offers that were already agreed upon were also sold AT THE SAME PRICE? Nobody gives you anything for free...