My husband and I did a similar trip in October but with twice as many days as you're planning. I do think that it's too much to cover in 10 days.
Consider either focusing on the north (Ljubljana, Istria, and Plitvice Lakes) or the south (Dubrovnik, Hvar and something else (Split? Mostar? Kotor?).
If you decide to focus on the north - look at flying into Venice or Zagreb. From Zagreb, there are great inexpensive airfares on Croatia Airlines from Zagreb to Istria (Pula or Rijeka.) You could rent a car there to travel in Istria, Ljubljana, and loop back to Zagreb for return trip.
For the south, look at flying into Split or Dubrovnik.
We did a round trip into and home from Zagreb. We flew Croatia Air to Split (2 nights), ferry to Hvar (3 nights), picked up rental car in Split (1 more night) to drive up the coast with an overnight stop at Plitvice National Park. From there we did overnights in Lovran (2), Rovinj (3), Ljubljana (3), and Radovijica, Slovenia (1 night for Lake Bled) then ended up with 2 nights in Zagreb. It was quite a lot of driving with some long days.
While it's not perfect, you'll get a good idea of travel times using Rome2Rio
It's all great- I just wouldn't try to do too much!