Hi David,
You are on the right track 🙂 In the "Advanced" Edit Menu of the input Arguments you can set the "Is Hidden Condition".
You need to make sure that each time you insert a first input value, your editor gets refreshed. So for your first input argument you toggle the "refreshes editor" button (see screenshot).
For the following dynamic input fields you can reference the first input value for example with
this.getARGUMENTNAME()
and add a condition to it.
I will attach a simple example feature definition here for you to test.
DynamicArguments.fdf
I hope this helps.
Cheers,
Hedi