> Maybe using an unnatural placement of )parentheses( could have worked as a non-conflicting indicator of italics.

Using different delimiter for opening and closing is a good idea on its own, too. I think it makes parsing simpler and unambiguous wrt nesting.

I've imagined something like this:

  `(monospace)
  _(underline)
  /(italics)
  ~(overstrike)

Probably looks a bit more distracting, though.