A nice way to increase the odds of the Redshirts rising up early is to start up a long war against one of your Italian brothers, raising their militancy. The Papal States is nice for this, since they start in France's sphere and can call them in to do most of the heavy fighting. Take Sardinia from S-P and Sicily from 2Sicilies (for the primary culture POPs in those states), and you should be good to go. I've never had to wait longer than 1850, and then after the Redshirts give you all of Italy, taking Lombardy and Venetia are easy.
If you don't want to form via Redshirts, then research high-prestige techs to boost you into GP status, then wage "Add to Sphere" wars on all the free minors until you've got a big enough team of spherelings to take on Austria. Then, if necessary wage humiliation and Cut-Down-to-Size wars on any Italian state too high up to sphere. Granted, this will take longer (and will probably use up a lot of infamy), but without a bit of luck you'd otherwise end up waiting most of the game for the other GPs to be distracted long enough for you to peacefully sphere all of Italy.