One van for 8 people would be a big pain. You’d need one that seats 12 or so, in order to fit in all that luggage. Driving it and parking it would be a nightmare. Two big vehicles will be better, but you MUST pack light. We are a family of 4 and we rent cars most of the time. If everyone brings carry on size luggage and one day bag, you’ll be fine. You will NOT fit 4 large suitcases in the back of a mid size SUV. Cars are smaller in Europe.
I would sit down with a map and reconsider the order of the stops. Switzerland will throw a bit of a wrench in your plans. It’s closer to Munich than Rothenburg.
You don’t want a car in any city in Europe. We rent cars, but never when visiting cities. We’ll stay outside the city and take the train in, or pick up the car on our way out of the city heading to our next destination. We prefer to stay in smaller towns. Unless you need/want to explore tiny villages, the train might be better. You don’t mention ages of your group, so that may be a factor. On trains you have to handle your own luggage, so again, pack light. So consider why you feel cars are necessary. Rentals are ridiculously expensive now (2-3x normal), then there’s gas. Just be sure you REALLY need them.