I’ll just add my two cents here. I’ve built (assembled) dozens of computers, both for clients and for myself. Things I’ve learned when working with CPU’s, coolers, and paste.
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You don’t have to remove the CPU when re-applying paste. Some pre-built computers use thermal pads instead of paste. Get rid of that. It’s a stupid idea. It is a good idea to lay the computer on its side, so you’re working vertically. It may sound intuitive, but you’ve be surpised how many people install a cooler with the computer standing upright. (I understood you to say you don’t have closed-loop liquid cooling, so you don’t need to worry about liquid spilling out of a reservoir.)
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You should have lint-free paper towels, isopropyl alcohol, and an air duster on hand.
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Start with the cooler. Wipe off as much old paste as possible. Clean with alcohol on a fresh towel, then dry it with short blasts from the duster. Repeat. Then repeat. The repeat again until it’s really clean.
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Once you’ve cleaned the cold plate on the cooler, DON’T TOUCH IT. A stray fingerprint can cause more problems than you think it could.
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Same procedure for the CPU. Get it really, really clean, and don’t touch it.
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As for the paste. I use Noctua NT-H2. There are other good ones, including some expensive pastes designed for extreme overclocking. You don’t need those. I want non-electrically conductive paste, with high thermal conductivity, so if you pick something else, pay attention to those attributes. Watch or read the paste manufacturer’s recommendations for use. Noctua recommended a single 3-4mm dollop on the center of the CPU die. It doesn’t look like much, but their paste has excellent spreading characteristics. Other paste manufacturers might have different recommendations, and those instructions might be different for AMD than they are for Intel. My 5800X3D idles @ 36°C and running the sim it stays right around 56°C (on a Thermalright Peerless Assassin). So it’s definitely working just fine. BeQuiet makes a fine cooler.
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When you attach the cooler to the CPU bracket, tighten the screws a little at a time, in a pattern, so you get even pressure on the CPU die. DON’T MANHANDLE the screw torque. You want the screws tight, but you don’t want to warp the die.
Once you do it once you’ll see it’s not hard to do. You just need to pay attention to the few things I’ve outlined.
Good luck. It sounds like you found the problem. Now fix it and go flying! 