I think I would use the seven nights in Slovenia differently. Ljubljana is lovely, maybe my favorite city in Europe, but it's small and there is not lots to see/do there. It's a good base for day trips. It's actually quite easy to day trip to Bled from there. Lake Bled is lovely, but the town of Bled is ordinary, a convenient base for the area but nothing special. I preferred the nearby old town of Radovljica (where I day tripped vs. spending a few nights in Bled).
You may not really need/want much time at Lake Bled itself. Plan out an itinerary of what you want to do in the area - Vrsic Pass? Lake Bohij? And plan where you stay based on that. I did a few shorter stays instead of having to double back to Bled. I stayed one night in Kobarid (not much of a town but a convenient place to stop).
You don't mention having a rental car for Slovenia, but you will probably want a separate car for Croatia or incur a steep one-way drop fee in Dubrovnik. You could take the train from Ljubljana to Rijeka and pick up a rental car there and drive an hour to Rovinj (this is what I did once). You could take a GoOpti shuttle or a bus (if there is a direct one) from Ljubljana and pick up a car in Rovinj, but there are fewer rental car options in Rovinj (more at the Pula airport).
I did not spent a night in Hvar Town - just a day trip. It was a lovely town but a little too busy and ritzy for me. I liked Korcula better too (did spend nights there). You could day trip by catamaran from Split to Hvar Town easily (no driving directly there - car ferry goes to Stari Grad instead). You could drop your cat in Split and not need one the rest of the way and use catamarans, ferries, and buses the rest of the way. But if you keep the car, you could drive from Split to Korcula (Vela Luka, other side of the island) via car ferry and from there on to Dubrovnik or from Split to Stari Grad (Hvar) and back to land from there via another ferry and down to Dubrovnik on land. Without a car, there are catamaran options.
I might put in a plug for Vis island as well - I visited last fall and it was by far my favorite of all of the islands I visited. It's hard to day trip there from Split and have much time - it's over two hours by ferry each way, a little less by catamaran. There is catamaran service between Vis and Hvar Town in September if you don't have a car, making that connection easier.
I don't think you need three nights in Split, either, unless you have specific plans there. It's a pleasant town and a good base for island hopping but once you've seen Diocletian's (ruined) palace (you walk through much of it), there isn't a whole lot else to see. A stop in Sibenik would be nice, I agree.
Staying overnight in Mostar would be ideal but you could day trip from Dubrovnik instead if you have dropped the car.