Show upcoming date only when multiple

What you're asking for is possible using Smart Date's Recurring formatter, which can work with dates that aren't recurring. You will need to install the submodule, however.

Here are the settings I would recommend for the formatter:

Force chronological: enabled
Recent Instances: 0
Upcoming Instances: 1
Show next instance separately: enabled

Based on my testing this works exactly as you describe.

I documented the steps to reproduce what you are looking for using an existing formatter. You haven't responded to the steps I took the time to document, so I can only assume you don't dispute that they work without the need for additional code.

If you prefer to user your own, custom formatter then that is also a completely valid approach, but as of now I see no reason to add it to the Smart Date codebase.