I agree that reducing the number of stops would be best.

In May, the boat from Korcula to Hvar seems to leave at 6:00 so if you could find luggage storage in Hvar, you could continue on to Split the same day.

The ferry to Venice doesn't run every day in May but is an option. However, you then need to travel from the ferry port in Venice to the train station so going to Bologna via Trieste may be easier, even though there are more travel segments. There are buses from Ljubljana to Trieste taking 1.5hrs if you change your itinerary but not sure whether that's an advantage. It will depend on whether there are direct buses from Rovinj to Trieste at that time of year (when we were there there was only one per day but rome2rio is showing hourly services but this will be seasonally dependent).

There are also food tours by boat from Korcula along the peninsula which may meet your desire to sample oysters and other food/wine from there. This may be worth investigating.

getbybus and croatiaferries are useful sites, we found.

Slovenia is beautiful but with your timeframes, I would spend those two days in Croatia or Bologna instead, making for a more relaxing holiday.

"I will have to backtrack to Trieste because my previous stop is Ljubljana"

Only if you reach Porec/Rovinj by bus.
By car, you can drive from Postojna either
via Lovran - Pacin
or
via Divaca - Koper
to Rovinj.
In both cases, you don't drive through Italy.

Happy to hear you were able to book the Plitvice Miric Inn. You will love it, especially the breakfast and dinner!

Thank you all for taking the time to write your suggestions. It will surely improve our trip.