What would Trump be obstructing if he canned Mueller? An open-ended investigation, which after 14-months has yet to connected him to a crime?
Makes no difference, as impeachment proceedings are political, not criminal. Are you willing to take the risk that Republicans would vote to save President Trump?
Trump has the authority to have Mueller fired, which in itself is not obstruction. Yet stopping the investigation would create the perception of obstruction. Google Saturday Night Massacre.