Fixing my refresh rate
It seems to me that Apple has the refresh rate locked at 30Hz for that type of resolution:
https://support.apple.com/en-us/HT206587


  The following Mac models support resolutions and refresh rates of up to 1080p at 60Hz, and 3840 x 2160 at 30Hz, over HDMI 1.4b when using the Apple USB-C Digital AV Multiport Adapter with macOS Sierra 10.12 or later:



iMac (2017 and later) 
iMac Pro (2017)
MacBook (2015 and later)
MacBook Air (Retina, 13-inch, 2018)
MacBook Pro (2016 and later)


Looking a bit further down on that link, it seems you can try to get it to 60hz by using the right cable and adapter:


  The computers listed above also support 60Hz refresh rate over HDMI when used with a supported HDMI 2.0 display, HDMI Premium Certified cable, and a compatible third-party USB-C to HDMI 2.0 adapter.