"with a 3 month notice, but could only quit every half year" I know little about German law, but this sounds insane. I also certainly wouldn't call it 3 months

"with a 3 month notice, but could only quit every half year" I know little about German law, but this sounds insane. I also certainly wouldn't call it 3 months notice, I'd call it 3-9 months notice. The real question is whether the company could only fire you at these two times of the year as well, or whether it only had to give 3 months notice. If the latter, you might find the courts agree that the contract is unfair if you took it to a tribunal.