There's no difference in capability between the HDMI and Thunderbolt/miniDisplayPort on the 2014 Mac Mini, which is to say that 60Hz is not possible. https://support.apple.com/HT206587
This is so annoying. I want to build out a machine with a ton of storage, and high res display. At this point I may be limited to buying a macbook pro, and leaving it docked permanently.