I actually built a PC for a mate with almost exact same hardware a couple of weeks ago. Temps were perfect from get go and this time round I used stock thermal paste from the cooler.
Unmount and clean off thermal paste. Reapply thermal paste. Carefully place pump back on thumb tightening each screw in a star shape. Do a few turns per corner to ensure even pressure. TBH, I didn't really know when to stop screwing as it didn't feel like it was getting tighter to an end point and I was also being aware of overtightening too. From memory, I think I did maybe 12-16 turns of each screw? Good luck!