The ferries and catamarans do have regular schedules. It's more a matter of the timing. They may not arrive at optimal times. When I visited Korcula (from Dubrovnik, heading north to Split), I had to take an afternoon bus because there were no ferries at that time. I arrive early in the evening, spent two nights, and had to leave at 6am on a catamaran the third day. So I really had only one day to explore.
I think there may be more options now than when I traveled there, but May is still not high season, so scheduled ferries/catamarans may still not be as frequent as in high season. You should be able to dig up the ferry, catamaran, and bus schedules from 2018 if the 2019 schedules aren't yet available. (The bus between Korcula and Dubrovnik uses the same short ferry to Orebic that you would use if driving - then bus or drive up the Pelješac peninsula to Dubrovnik.)
Split is one of those places people are kind of...split on. (Sorry.) Some people absolutely love it. I did not. I think my expectations were too high, and the weather was lousy (pouring rain half of the time) by the time I arrived in mid-October. Perhaps I would have liked it better in better conditions. But the city just didn't grab me for some reason. Didn't hate it but didn't fall in love with it. (Like I did with say Ljubljana.) Wasn't "wowed" by Split the way I was by Dubrovnik, which is also a big tourist trap. The town built partly over Diocletian's Palace is truly unique but not something I needed to spend a lot of time in. (Some of it is filled with retail shops.) If you spend much time in Split, plan some day trips (e.g. to Trogir) - and if you love Split, just stay there if you want to, but you can take your day trips if you want to. I like making contingency plans.
I don't see Plitvice Lakes National Park on your itinerary. It's truly an amazing park, with dozens of little waterfalls, one of the highlights of all of Croatia in my view. It's also extremely touristy and mobbed mid-day, but I still think it's worth seeing if you can visit when the park opens at 7am (by 10am, it gets busy, even in May). So it's best to stay overnight at a park hotel or very close to the park to make it worthwhile. I wouldn't try to do a day stop on the drive where you would arrive late in the morning just as it is getting so crowded.
I'd stop in the town of Sibenik on the way south from Zadar for sure - lovely town, some beautiful churches too. (There's also another nice park, Krka, near Sibenik, but I'm not sure how interested in parks you might be.)