Split to Plitvice to Ljubljana or Zagreb and beyond.

Or let someone else plan it and set up the transportation for you. I have a contact in the region that can handle all of it for you and might be able to help you decide what to do in detail. PM for the contact information if you are interested.

I think for a group of 8, James's idea would probably be affordable. I'd certainly recommend exploring it.

There is public-bus service from Split to Plitvice and from Plitvice to Zagreb. I have the impression (I haven't researched this) there are more buses heading to and from Zagreb than to and from Split. I've used the Zagreb-Plitvice buses myself, and they were reasonably frequent as of 2015. What I don't know is how convenient the bus pick-up point would be to your Plitvice hotel. I recall being a bit nervous about finding the right place to wait for the bus. (I did find the bus shelter.)

Should you end up on Croatian buses at some point, be aware that it is legitimate for the driver or an assistant to charge a small fee (it was the equivalent of about $1 US back in 2015) for each piece of luggage stored in the hold of the bus. It's an unusual policy but not a scam.

Not sure if this is of any help, but if you decide to visit both Zagreb and Ljubljana, there is a train you can take from Zagreb to Ljubljana and vice versa. In 2019 it was $9.00 per person one-way. It takes about 2 - 2.5 hours through pleasant scenery.

There's also bus service between Zagreb and Ljubljana that doesn't take much longer than the train. It's not that the buses are so fast, but rather that the trains are slow. In Zagreb the train station is somewhat more convenient to the old town if you are on foot, but there are trams from the bus station as well as the train station, so the difference isn't really significant. The bus station is a happening sort of place, whereas the train station (as of 2015) looked disused even though it wasn't. In Ljubljana there's really no reason to prefer one option over the other.

The bus station is a happening sort of place, whereas the train station (as of 2015) looked disused even though it wasn't.

Well, I can't really agree with that. The bus and train stations are both pretty active. We have taken trains to and from Zagreb in 2019. In both arrival and departure, the train station is well-staffed and plenty active. For the train east to Beograd, there were several dozens, perhaps even 100, people waiting. We obtained our ticket the day before from a station agent. The train to Ljubljana is similarly well run. The bus station, which is probably 500 M from the train station, is very active as well, with many buses running from early morning to later.