This looks like a very nice northern Italy itinerary, especially for a family with teens. The pacing is good and you are not trying to cover too much.
For Lake Garda in August, expect heavy traffic and parking pressure, especially around the east shore. Torri del Benaco is lovely, but plan lake days early and don’t underestimate ferry use — ferries can save time and stress. From Torri, easy highlights are Malcesine / Monte Baldo, Bardolino, Garda, and a relaxed lake swim day.
For Cortina, 4 nights is excellent. Try to start hikes early because afternoon thunderstorms are common in the Dolomites in summer. Great family options: Tre Cime di Lavaredo, Lago di Braies, Cinque Torri, Lagazuoi, and Lago di Sorapis if your boys are strong hikers. Check parking/reservation rules for popular spots before you go.
Dropping the car at Venice Airport is the right move. From the airport you can take the Alilaguna boat or bus depending on where your Venice lodging is. The best to visit Venice attractions from 6:00-9:00, go away and come back after 18:00, some museums are open until 23:00.
Only thing I’d watch: August means HEAT in Bologna/Verona/Venice and CROWDS at Garda. Build in slow mornings or swim/rest time — with two boys, this itinerary will be much more enjoyable if it doesn’t become a daily sightseeing march.