Hey,
you can go to Rijeka by train (more beautiful route than to Zagreb IMO, but Zagreb one is near Ljubljana also quite pretty). Then from Rijeka or Zagreb go by bus to Plitvice lakes, really nice. On the way to split you can then make a stopover in Zadar, there should be direct bus from Plitvice, from Zadar Split will be not far anymore.
If you go to Plitvice, definitely opt for the long hiking route (18km if I remember right). This one is usually not busy, while the main area (direct after the entrance) is overtourism at its best.
Thank you so much yr advice Martin! We're actually on our way to Piltvice now and will explore the area tomorrow. Definitely prefer to avoid crowds when out in nature so really appreciate the advice re the longer, less travelled trail!