Northern Italy is fantastic for cycling. It was late September when I was there and the weather was perfect, dry, warm and not too hot. Lots of people cycle and there is excellent provision for cyclists. Hotels are cycle friendly and outside the main tourist spots affordably priced. If you search around there are some iconic places to stay in castles and monasteries etc. There are quality long distance cycle paths often along disused rail tracks and beside aqueducts/canals with good surfaces.
I'd suggest your first section to take a slightly northern route to/through the Dolomites if you're up for it. The landscape is absolutely stunning! The gradients are not particularly taxing - you'll drop down to Treviso then Venice well before Stelvio. By contrast the route nearer the coast is pretty uninteresting.
One thing to watch out for is most shops other than big supermarkets (of which there aren't many) close all afternoon, and petrol stations sell *only* fuel, no food or drinks. There are a lot of public water points (shown on OSM mapping) which is handy, though occasionally they require a water card.
Oh and don't take your bike into Venice, the fine is hefty.