From Illyria it is still a weeks march to northern Greece, the Spartan kings would have a week to prepare. From Epirus we could be in the Peloponesse by that time, threatening Sparta herself. Also Epirus could be easily supported, as it is right across a small stretch of sea from Italia and could be easily maintained as it is surrounded by a wall of moutains, just like Gallia Cisalpina. Instead of conquering Illyria for the money right now, I would suggest going for the strategic value now and conquer economically valuable provinces when we are actually winning this war.