I’d strongly oppose such a change, as it’s visually noisy. The outline sufficiently conveys a button, and now that the icon changes more for each post type, the dynamic nature of the button is illustrated more vividly:
What Discourse could do, for new users, is pop up a small education panel (unless you opted out at sign up) that explains what the button is, and what it is for, cc @tobiaseigen. It’s another example of just-in-time education, though IMO a bit of a weak one as this is meant to be advanced user functionality.