No you don't decrease the voltage. Just increase the LLC and keep the voltage the same. And you probably didn't increase the vcore enough to remove the error. If you're at the edge, sometimes it requires a substantial increase. So it's better to just raise the LLC and keep the voltage constant.I tried increasing voltage didn't fix, atm I use LLC6 with 1.20v in BIOS resulting in 1.14v-1.15v in full load, you think I should increase to LLC7? But I would like the same load voltage so I would have to reduce the vcore in BIOS a bit to have the same 1.14v at load, definitely reducing ring ratio and increasing CPU vcore didn't fix. I have a big stuttering when the error occur while in game but I can still play fine for a long time before the 2nd WHEA error. It doesn't crash the game or anything, just a stutter.
You can't have your cake and eat it too. If you want to remove the parity errors, you have to pay the entrance fee. Or lower the overclock.