When you .setConfirmationMessage(message)
on form submit, the current message still gets displayed and your new one won't show up until the next form submission. Seems like the order of operations is - 1) submit values of form to spreadsheet 2) show confirmation message 3) run "on form submit" functions.
So, we can't conditionally set and immediately display a custom message for the current user's response based on what their answers were. The confirmation message is "front-loaded" in a sense.