To undervolt you need ucode 104. Some motherboards have an option built in to use the 104 ucode without changing bios, but its... old... and kind of buggy... had the problems that caused the RL panic. And in my case at least, if I activate 104 I have to also turn intel profile off for the undervolts to work, so I don't do it.
If you don't have a mobo with that feature the only option is go back to an old bios version with that old ucode, if it exists. And who knows how it would react to such new intel firmware. Might be a bad idea.... Since ucode 0x123 (including the one you have now) you should be able to undervolt via the loadline from the bios. Its not the best way, but its a way that isn't dangerous at least. You can only do so much before cep gets in the way. You can turn cep off but you need to turn intel profile off first. I'd say thats still probably safer than going to 104, but who knows. I'm biased cause my first 14700kf died on 104 and my second started acting weird af when using it. So I avoid it like the plague now.