While Steves and others often recommend starting north and increasing the cultural "temperature" as you go south, I agree in your case with the weather concerns about starting south, and I'd recommend Amalfi Coast/Capri during the Rome segment, and then head north, perhaps Chianti trail and the hill towns if you enjoyed Tuscany before, maybe Cinque Terre while you're in the neighborhood, and then Venezia for at least a week, very romantic, try for Locanda Orseolo if you don't already have a preferred place. You might check the weather for the lakes area at the end of April; it might be warm enough to end your visit on Lake Como.

I spent an Easter week (April) in Rome. It rained the entire week and was pretty cold. There were plenty of people everywhere but it was not so crowded that it was unpleasant.

We also plan to be in Italy next year but the month of May. We start the last weekend of April in Rome, then a week in Assisi, two weeks in Venice, a week on Lake Como and then another five weeks in Switzerland.

We will be renting vacation rentals for the whole trip and will do day trips or overnights from these home bases. We plan to fly into Rome and out of Zurich.

We have been to Rome, Assisi and Venice a few times before and always find ourselves wanting to go back. This will be our fifth trip to Italy.

We have been to both Venice and Lake Garda in April and the weather was perfect (and Garda was not in the least crowded). I think you have a good plan.