So, I just stumbled upon an amazing heatsink while replying to a comment below. I use a painkiller carbon tablet pipe [0] that fits (barely) in my poe hat fan hole.

It brings the idle temperatures down to 42 degrees C, and sysbench never goes above 61 degrees.

Image here: https://imgur.com/a/Cr38Abz

[0]: https://www.apotea.se/treo-brustablett-500-mg-50-mg-20-st