My CPU Cooler is a Corsair Hydro Series H115i
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and
the thermal paste is the Arctic MX-4 which can cost between $5-10
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The Arctic MX-4 is actually amazing value, good choice there.
As for the CPU liquid cooler, unless you're tight on case space and mounting holes, I have no idea why you'd pick the overpriced H115i when for slightly more money ($160 vs $150) you can have the Corsair H150i Pro RGB which is a 360mm triple fan AIO, vs your 280mm dual fan AIO.
I'm sorry, but this is just overpriced. You're basically paying for the rainbow puke (as Steve calls it), no idea why people pay that much for a few blinking LEDs.
As for your thermal paste concern, I have seen this happen before in graphics cards, not in CPUs, but it can apply to either.
What happened is that you were using bad quality thermal paste, thus it didnt live very long.
Usually those are TIMs (Thermal Interface Material) that are included in 3rd-party coolers and sometimes even in CPU coolers that come with the CPU (Intel/AMD).
I assure you, this doesnt happen with quality thermal paste that you typically buy separately, such as your MX-4.
Trust me, I've used tons of different TIMs from different brands,
Noctua tends to be among the best, but not quite the best. Arctic is the best value for money, especially the MX-4.
And btw, what you said about replacing it every 2-3 years, is just false. The fact, is, there isnt any amount of time that a user should replace their CPU's thermal paste. It all depends on usage, quality of TIM, the CPU and the cooler that is being used, etc.
I can assure you that I've personally used machines for an outstanding 6-7 years, daily usage, and the thermal paste which wasnt replaced, still had life in it.
Let that sink in.