Would it be suitable for a really slim mini itx system? Particularly one with a Ryzen APU?

It works. It is just very very very loud. I recommend other low profile ones, these are meant for server racks where you care more about space than noise

Is it quiet, and does it keep the CPU cool?

It is as loud as your vacuum and will overheat if you try to make it any less loud. I have it as a brick now. Don't buy it if you care about noise. It is only good for if you leave it outside or underground.

Questions & Answers: Dynatron A18 AMD AM4 Processor 70 x70 x15mm PWM blower with Copper fins active Heatsink for 1U Server up to TDP 95 Watts CPU Cooler

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