Wow that’s a nice case with big fans!
I really don’t think that doing a poor job on the thermal paste makes a big difference. I’ve see plenty of YouTube about that to indicate the it really doesn’t make a big difference on how you apply it or how much. As long as you applied some, you should be good in that area.
You mention at times the CPU package can spike to, you typed 10C but I think you meant 100C, which are shutdown levels. You should never be reaching those temperatures. The CPU will throttle down to protect itself and will start processing slower and you may see stutters due to that.
At this point, I wouldn’t mess with voltages. By default the voltages will be set to good values.
The problem you may have is the amount of intake air versus exhaust and where your water cooler is installed.
You had mentioned that it was top mounted and set as exhaust, which as I mentioned I think it’s bad to try to cool your water with hot air from inside the case. The cooler should ALWAYS be an intake of cool air from the outside.
But you also need to determine how your four 200mm fans are installed and if you had to remove some what your overall intake/exhaust configuration is. From looking at the image from your case, I would imagine that the front is intake, the top and maybe the back also exhaust which would make your case a negative pressure case and would have the tendency to suck in air from available openings in your case and eventually cause a lot of dust inside the case.
You always want a positive pressure case, meaning you always have more intake filtered air that exhaust fans and that would create a positive pressure case and blow clean air our of available holes in the case and keep the in side clean.
And positive pressure will also keep the inside of the case cooler and and your components cooler.
I think you mentioned that you installed your 240 radiator on top in exhaust mode. So I guess you probably had to remove the top two 200mm fans. So you swapped out two 200mm fans for two 120 mm fans?
I think you intake and exhaust fans may be out of whack and why you are hitting 100C, which you never should, and you have a great case, so I think the fans or the controls of the fans are not setup properly.
If you want we can get on a Discord and talk more about it. My first setup sucked and now it’s great and the only difference was the case, the cooler and the fans.
You have a great case, so we should be able to make it work great. I think it’s just a matter of ensuring the fans are working properly. All cooler basically work the same, so that too we can make work great.
Tonight I did a 3 hour group flight and my coolant temperature never went above 38C. That’s another thing I had mentioned in a previous message, cool based on your cooler and not your CPU or CPU Package, by then it’s too late and everything is already too hot.
Like I said, if you want we can chat on Discord. Much easier than going back and forth with messages.
Edit: Wow super long message 
Any of those cards would be a huge bump from my rtx2060super providing I am already gpu limited on a UWQHD monitor.