Dubrovnik/Korcula/Hvar

We visited Hvar as a daytrip from Split so we didn't stay overnight and did not visit Korcula so I can't really help with that. However, I can recommend the places we stayed if that will help.

Dubrovnik - Karmen Apartments
https://www.karmendu.com/
Lovely apartments, nice host, excellent location just inside the Ploce Gate

Split - ID Regal Residence
https://id-regal-residence.splithotels.net/en/
The only negative is that the rooms are located on the 4th floor, and the building doesn't have an elevator.
The rooms are lovely, and the included breakfast is at the Bokeria Restaurant. Absolutely delicious breakfasts!

Plitvice Lakes - Miric Inn
https://www.plitvice-croatia.com/
Just 5 minutes from the park's entrance. one of the best breakfast buffets we have had anywhere

Zagreb - Jagerhorn Hotel
https://www.hotel-jagerhorn.hr/en/mobile-en/homepage/
Very convenient location, nice but small rooms, very good breakfast

Korcula was our favorite island of Croatia, so my vote would be for two nights there. I highly recommend Apartment Marina if she has availability. Killer view, gorgeous apartment and fabulous service from Marina and Sascha. https://www.booking.com/hotel/hr/apartment-marina-korcula.html We took the ferry from Orebic to Korcula, after a day of wine tasting on the Peljesac Peninsula, so there was no need for advance reservation.

As for Plitvice, our stay at Zrinka House was great. Large room, terrific breakfast, and Zrinka and her daughter Barbara packed us a lunch for the next day. Five star service all around! https://www.booking.com/hotel/hr/guesthouse-zrinka.html Barbara warned us of speed traps, cameras, and a high police presence on the drive from Plitvice to Zagreb. Forewarned is forearmed. :)

RS generally suggests just picking one or the other, and I would too. Just keeps things simpler. Maybe if you had more days, but I wouldn’t do it for a one-nighter. And really, my observations on this forum is that no one particularly places one over the other. They just have different vibes, you can’t go wrong. I did not make it to Hvar, but Korcula was amazing. Three nights there would be perfect imo.

Thank you so much, these are all great suggestions. That hotel in Zagreb we are booking and the apartment suggestion in Dubrovnik we have sent a request in. Looking at ferry times it looks like we need to do at least Saturday night stay in Hvar as they have the best morning ferry rides to Split. Unless you all are aware of them adding more times as it gets closer to that month.

Since you sound like a young couple, I would just stay on Hvar.

I was torn between Hvar and Korčula.  I chose Hvar because I wanted to do a boat trip to the blue cave and also wanted to check out the castle.  That boat trip was a highlight of the trip for us, btw.  I’ve read some great blogs and trip reports of kayaking and relaxing on Korčula that also look amazing.  Either way you can’t go wrong. Depending on how the timing works, you could potentially stop at Korčula for a few hours before continuing on to Hvar.  Or if you don't mind a faster pace, split it and stay on both.  No need to book catamarans now.  Some of them will show you how many seats are left so you can have an idea of how full they are.  There are three companies, most likely you would choose the one that has the best time for you.  There is usually a subsidized route each day that is about half price, so look for that if you are on a budget, but these may sell out earlier.  RS recommends getbyferry.com to see the options, then book direct with the company with the time/price you want.  

In Dubrovnik we stayed with Jadranka Benusi.  In Hvar we stayed at Apartments Mare.  In Plitvice we stayed at Miric Inn.  It just so happens that all these places are in Rick's book.  I booked them all on booking.com, but later I was able to cancel my booking with Jadranka on booking and coordinated with her to book directly with her at a slight discount.  I'm happy to share more info on any of these places if you are interested.