Hi Nick H_553,
The resolution and refresh rate supported for the external display depends on how many displays you are connecting and whether the display on your Surface is on or off. It also depends on which processor you have.
To change the Refresh rates on Display settingsFrom Start menu > go to Settings > then select Display > scroll down and select Advance Display Settings > Display Adapter properties > Monitor > select on Screen refresh rate drop down.
See this article for Surface Go resolution and refresh rate.
Connect Surface to a TV, monitor, or projector
Regards