Need Itinerary HELP with transportation questions

While you can easily travel between all of those places by bus or catamaran, if you have too much to comfortably carry, I would rent a car at least for the Zagreb to Plitvice to Split portion.

The Zagreb to Spilt train does stop in Licko Lesce about 20 kilometers west of Plitvice, but I'm not sure what type of transportation might be available between the train station and the park. Unless you can find reliable transportation between them, using the bus that stops right by the park or a rental car would be easier.

Not sure when you travel on to Dubrovnik, but keep in mind that the Jadrolinija catamarans stop running at the end of September while the Krilo catamarans continue running into October. It means that after September your departure time options and opportunities to stop for a few hours at islands along the way are reduced.

I really enjoyed my day trip to Montenegro (Atlas Tours), so I would consider that as one of your options while in Dubrovnik.

We just got back and did a similar trip for 8 days in Croatia. Zagreb for 2 nights, rented car to Plitvice Lakes (arrived afternoon to avoid some crowds), stayed at Plitvice Lakes National Park lodge (dated but cool), left in rental car next morning for Split, dropped car at airport, ubered to Airbnb for 2 nights in a split, Krilo ferry to HVAR, Korcula (1 night there, which we would have loved to do 2), ferry to Dubrovnik for 2 nights, then on to Venice. Me and wife in our 50’s and 20 something daughter. We were under strict orders to pack light because of all the transpo and stops. Sounds like this might not work for your situation and all the gear you have. Only reason we got a rental out of Zagreb was because we intended to hike Lower Lakes Plitvice late on the 1st day and the hike the Upper then next morning and I did not want to wait around all day for the afternoon/evening bus to Split (getting to Plitvice by bus early in day is easy but then you have crowds). The car rental cost was $350 for that one day (“Your coverage, Sir, includes all the metal around the car but if you want fender, wheel, tire coverage, that is extra!” Believe me, I researched everything on rentals in Croatia and I still got taken to the cleaners mostly because we picked up in Zagreb and dropped off in Split). But driving allowed us to store everything in the car and go at our pace. The drive from Zagreb to Split was wonderful. Modern freeways, easy to understand signs, etc. Highly recommend it for non-financial reasons.

Lastly, let me just reiterate the Rick Steve’s travel mantra; “There are those that pack light and those that wish they did”).

We spent over 3 weeks in Croatia & Slovenia in Oct 2013. Picked up car in Zagreb, drove over Julian Alps, down to Istria, over to Plitvice, returned to coast, drove Hvar Island & town and dropped car in Dubrovnik. No problems with traffic or parking except in Split. Recommend staying in Dubrovnik suburb as shlepping luggage up & down Dub hills would be too much work. As we are in our 70's, we try to travel lighter each year, but sometimes you just need "stuff". The trip south of Split was enjoyable as we avoided the highways as much as possible. We have spent beaucoup time in Venice, but do not think Dub is similar except for tourists. Stayed in Dub, went down to "main street" in mornings, explored away from that area after buses arrived, then returned to "Stradun" in evening when locals came out to stroll, dine & party. Relaxing and very enjoyable. Buses & reasonable taxis to airport (south of town). Definitely recommend renting car Zagreb to Dub as easiest & most efficient way to travel in your case.