No. It is not realistic to “see” Tuscany and Umbria without a car, or a friend’s car. You can see Florence and a few other towns. That yes. There are commuter trains from Florence to nearby gems. So, to answer your question precisely. In Tuscany, stay in Florence. It’s not as charming as a town but it makes things easier. You could also select Siena, but you’d need to get past the trepidation. Some of the back roads are narrow. Lighting is poor at night. In Umbria, stay in Perugia or Orvieto (the latter is closer to Rome). If you want to avoid renting a car, you just need to narrow your choices. For example, stay in Florence but pick a few towns in commuter train range, including Siena. If in Perugia or Orvieto, take the train (two switches) to Gubbio. Getting a guide for day-trips is possible. Finally, the travel times (train or car) are comparatively short. The fast trains from Rome to Florence, for example, are 90 minutes. Florence-Siena (slower train) is also 90 minutes. Don’t let yourself feel overwhelmed. Pick a place, settle in, go from there.