Hey, if you use the days in Split to explore close by cities and do some island day trips you could spend your last 5 days staying 2 days on one of the islands like Korcula and then from Korcula going to Dubrovnik for 3 days or 2 days if you need to return for your flight from Split.
8 BEST DAY Trips from Split (Guide) By Bus or Car

BEST Islands to visit by ferry from Split Croatia (Guide to Island Hopping)

Here are a few articles for what I mentioned and also other options, if you rent a car you could even do a road trip from Split - Dubrovnik and can also stop at Korcula etc.

How to travel from Split to Dubrovnik (Bus, Ferry, Plane, Car)

Korcula guide, an island not to miss in Croatia

Road Trip from Split to Dubrovnik (Best stops along the way)

Also from Split if you will have rent a car you can visit Plitvice Lakes, its a bit further away but worth the drive.

How to get to Plitvice Lakes from Zagreb, Dubrovnik, Split, Sibenik, Zadar (Guide)

If you will need any city guides, restaurant guides, sightseeing, beaches for your mentioned locations check this subs Megathread here which has all links compiled or this CROATIA TRAVEL GUIDE.