I would visit Otzi during the days you are in Ortisei. Bolzano is about 50 min drive from Ortisei. By doing so you avoid having to deal with having your luggage in the trunk while visiting Bolzano. Doing otherwise would basically mean that you will end up taking the freeway to Venice, because by the time you are done with Otzi you won’t feel like taking the longer mountain route.
If you visit Otzi during your days in Ortisei, then, on the last day, you would simply go from Ortisei to Venice through many of the scenic roads through the mountains. There are several routes you could take, and the drive is from 3.5-4 hours and up, depending on which one you choose. If you want you can swing by Cortina as well without adding much to the time. Obviously by the time you consider the pit stops for photos and snacks, it will take you way longer than 4 hours.