Almost-final 3-week itinerary

Be sure your rental company allows you to take the car into Bosnia-Hercegovina.

While islands sound a bit more romantic in the imagination, you’ll still see plenty of beautiful coastline in mainland Croatia.

Thank you both for your responses!

"June 1: Drive from Mostar to Split, via Peljesac peninsula, Ston and Mali Ston."
Maybe you are taking the car ferry from the peninsula (from Trpanj to Ploce)? If so, you might consider driving up the south side of the peninsula, and through the small one-lane tunnel.
We took that route in 2017 when we did Dubrovnik-Split-Plitvice Park-Rovinj. The tunnel is mentioned at end of Part 1 of trip report posted on the RS Forum: https://community.ricksteves.com/travel-forum/trip-reports/croatia-road-trip-aug-2017-dubrovnik-and-drive-to-split

Jeff B - thanks for the tip! I very much enjoyed your trip-report and found the details about driving routes etc extremely helpful.

On June 13 which hill town were you thinking of? If Motovun, park at the bottom of the town in the lot and walk up. But be warned it is a long walk up. It is very hilly and the streets are old, large cobblestone. Wear sturdy shoes. Don’t go if it is raining. My husband drove up and parked near the cemetery. The signs clearly read no parking, he reads Croatian. Anyway, surprise, we got a parking ticket. The ticket stated that if you paid immediately it was 1/2 price. We paid at the booth in the parking lot. It wasn’t much, maybe $11. Motovun may have been an artist colony back in the day but now it is mostly empty houses which many are for sale. Most of the artist have left and not many craft shops. Believe me, I looked because I am a shopper especially of local artisans. We walked the ramparts, had coffee at the top by the castle which had beautiful views, but not much else. We then drove to Poreč where we ate a lovely lunch by the water and enjoyed a self guided tour of the Basilica. By the way, Poreč is flat.

Barbara, lol! My friend refused to walk up the hill at Motovun so she paid for the bus up for both of us! And yes, parking tickets are half price if you pay online quickly.

https://community.ricksteves.com/travel-forum/trip-reports/croatia-parking-ticket-solution

But this link is where to pay them - OR to pay your parking ahead of time OR add time if you find you are going to need it. https://paydo.hr/

Thanks for the heads-up about parking/walking in Motovun. We were planning to visit Motovun, Oprtalj, Groznjan, but are open to suggestions!

Day 3 seems pretty ambitious to me. Presuming you have a car, it could be done, but only in the most cursory way. Maybe do a little more research and pick just one or two? You seem to be using Rovinj only as a place to sleep. It's an interesting little town on its own. Try to find time at least to spend an afternoon/evening. Honestly, I'd skip Motovun. It was boring and crowded with tourists the first time. The second time I took my husband, who had not been there, to give it a second chance. It was still boring and crowded. If you do go, heed the warnings above. They are spot on. Personally, I don't understand what people see in it at all. We enjoyed Groznan and Poreč the most of several hill towns.

Thank you Wanderlust 58! By "Day 3", are you looking at our third day (day-trip to Kotor) or June 3 (Trogir, Salona, Klis)? I think probably the latter. Thank you for pointing out that that will be too much! We will indeed have a car, but I will try to pick two of these places to visit instead of all three.
For Rovinj, you are quite right that it looks like we aren't spending any time actually enjoying Rovinj. We had thought, since sunset will be quite late in June, that we would spend the late afternoons and evenings in Rovinj (4 or 5 PM until dark and also have dinner), but in reading everyone's comments, it seems like we might be best to cut our day of visiting hilltowns to a half-day.
I really appreciate your input. Thanks!