How to get 60Hz on a 4K Monitor using a 2018 Mac mini
Get this USB-C (Thunderbolt) to DP cable.

I use it myself to run a 4k Dell monitor with 60hz, and it works flawlessly. You are right that most HDMI adapters don't support 4k with 60hz, although the official specification supports it theoretically from HDMI 2.0 onwards. Using the mentioned USB-C to DP cable solved this issue for me.