In LaTeX that would be close to option 2:

    e^{|x|}
    f_{i(x)}

Except with {} for grouping, which I think is a good thing, as {} are never rendered, unlike parenthesis.

I haven't used Typst much, so I am a bit wary of typesetting engines that are "too smart": It can be problematic when introducing new notations, which is quite common.