Help with Croatia/Slovenia trip

What kind of travelers are you? To be honest, this looks pretty exhausting to me. As soon as you settle into a place, you're going to be leaving again. And travel times are slower in this part of Europe.

There are a few places on this list that I haven't been to, but you for sure will want more time for Dubrovnik and Venice (minimum 3 nights each). I personally would narrow this list down so that you have a more relaxing, enjoyable trip.

I’m of a similar disposition, but I have the same question—is this the way you like to travel? Have you done so before? Otherwise it’s just a personal preference. I found Croatia to be very much a slow down and smell the roses kind of place, and I always did more nature activities while I was there.
Your transport details are needed though, as if this is all but public transit some might be a bit tight.

Here is the same scheduled but I added in our transportation plans. Currently we are viewing Split and Zagreb as quick rest overnight stops on our way to our next destination. Also using Zagreb to ditch the Croatian rental car to avoid the fees for leaving it in Slovenia.

Day 1 – Fly into Dubrovnik

Day 2 – Dubrovnik

Day 3 – Day trip to Mostar (Bus?)

Day 4 – Ferry to Korčula

Day 5 – Ferry to Hvar

Day 6 – Hvar

Day 7 – Ferry to Split (viewing this as a quick stop mainly to launch us to Plitvice the next day)

Day 8 – Drive to Plitvice Lakes and Zagreb (Zagreb viewed as another quick stop mainly to launch us into Slovenia the next day)

Day 9 – Train to Ljubljana

Day 10 – Ljubljana

Day 11 – Ljubljana and Postojna Cave (Drive)

Day 12 – Drive to Lake Bled

Day 13 – Lake Bled and Vintgar Gorge (Shuttle)

Day 14 – Drive through Vršič Pass and Bovec?

Day 15 – Drive to Piran

Day 16 – Piran and ferry to Venice (flight leaves from Venice so quick explore there)

Day 17 – Venice

Day 18 – Depart

I would pick either Korčula and Hvar. If you are getting to Dubrovnik after a long travel day, I would omit the Korčula day and add it to Dubrovnik.

I agree, pick one island and that helps tremendously!
You could consider a slight splurge and pay a driver to take you to Mostar and then drop you at the Korcula ferry point (Orebic), maybe see Ston en route also.
Or, eliminate Hvar and stay in Korcula. Catamaran schedules are on Krilo and Jadrolinija websites.

I agree with the others that you may be trying to do too much in too short of a time. I spent the better part of the day in Split and never ran out of things to do. You could take the Flix bus between Croatia and Ljubljana. It's cheap and it's quick. You could also take the Flix bus to Venice. I think you could just stay in Ljubljana and then do day trips from there to Lake Bled. There is at least two days of time you could spend in Ljubljana, and all of it could be done by walking. A trip to Piran will be nice, but you will be disappointed if you try to make it a whole day. Lake Bled and Vintar Gorge could be done in the same day. You probably will have to rent a car to go from Ljubljana to Postojna because I don't think public transportation is quite as timely.