Is harmony really that important?
Yes, it is. It’s impossible to become a professional composer, if you don’t study harmony first. Preferably classical harmony (with the Roman numerals). You will have to take lessons though. Do not study harmony on your own. If you want to get a head start on other students, then you could buy the excellent Harmony and Voice Leading by Edward Aldwell and Carl Schachter, but you’ll always need help.