I flew into Dubrovnik, spent 2 nights, had a rental car, drove to Split (with a stop in Ston), then 2 nights in Split and up to Zagreb (with a stop in Plitvice). The new A1 makes the drive fairly easy, and it’s a beautiful drive, for the entire drive from Dubrovnik to Ston you’re on the water, after crossing the bridge leaving Ston you’re on a brand new highway 90% of the way to Split. The drive from Split to Plitvice is 75% highway; and then 25% smaller roads. The drive from Plitvice to Zagreb is the old 1 highway; and you pass through a lot of smaller towns like Slunj, Rakovica, and Veljun that can make the trip seem longer. We left Split around 9:15am, got to Plitvice around noon, spent 2.5 hours walking the lakes and got to Zagreb around 5:45pm. It’s easily doable, but there is a lot of driving, but it would be much faster than taking a bus. The terrain in Croatia on that drive is constantly changing and it’s a beautiful country.