It took us 9 nights just for the Dolomites, Venice and Ljubljana. So—yes— your itinerary is quite hectic.
Note that each time you change locations, you’re actually using up most of one day.
Did you know the train ride from Munich to Ljubljana takes a minimum of 6 hours? There are flights that take 50 minutes.
I think you should forget about Plitvice Lakes National Park from Ljubljana as you already have enough to see since you’re cramming Slovenia into just two days ( three nights=two full days). You would arrive at Plitvice Lakes too late to miss the hordes of other tourists who swamp the park by mid-morning on a daily basis and you will not have enough time to actually enjoy the park before driving 3.5 hours back to Ljubljana. Honestly, It’s similar to tourists booking a daytrip on a bus from San Francisco to Yosemite Valley for the day and then wondering why it seemed so rushed and exhausting. Save it for another trip.
In the Dolomites, Selva di Val Gardena or Cortina d’Ampezzo make for better bases than overrun Ortisei.
Selva di Val Gardena has access to less crowded hiking trails and quicker access to superior hiking in the portions of the Dolomites east of Selva. A car is necessary to access hiking in areas such as Cinque Torre ( “Five Towers”) near Cortina d’Ampezzo.
You will need to take a 3-hour bus from Ljubljana to Venice or spend 6-7 hours on a train for the same route. From Venice, you can rent a car or take the 3-hour train to Bolzano. Add another 1.5 hours minimum for the transfer to a bus and then the ride from Bolzano to Ortisei. OR, you can drive up from Venice to Cortina d’Ampezzo in 2 hours. I highly recommend this. Not only would it relax your itinerary— it would take you to the eastern Dolomites where the real spectacular scenery like Cinque Torre and Lake Braies are located. Either way, getting from Ljubljana to the Dolomites is going to take the better part of a day.
In Europe, the car rental companies charge an exhorbitant supplemental charge— usually hundreds of euros—for picking up a car in one country and dropping it off in another country. If you opt for a car rental, pick it up in Italy and drop it off in Italy. www.AutoEurope.com brokers car rentals for the big companies and they have rental locations in Venice and Bolzano, among many other Italian cities.
You could pick up a rental car in Venice and drive to Cortina d’Ampezzo or a village like Colle Santa Lucia for four nights. Then explore the Dolomites before returning the car to Venice. Finally, take a high-speed train from Venice to Rome in just 4 hours.
Carpe Diem!