Help- Traveling from Zagreb to Dubrovnik, 7 days

Do you mean you have 8 days (or 8 nights?) between Zagreb and Dubrovnik? Or does your 8 days/nights have to include arrival day (don’t drive that day) and time in Dubrovnik?

Start by choosing priorities. Sounds like you want 3-4 nights in Istria. Pula makes a great day trip. Motovun and Hum a day unless you have something special planned. Another night for wineries? There is much to see and experience in Istria.

And you optimally need an overnight for Plitvice. Maybe a night for a stop between Plitvice and Dubrovnik. I assume you will have a car.

Start by making your priority list and begin to estimate how long you want to spend. Might be different for me.

We need more info--are you going to spend any time in Zagreb? Would you be able to drive upon arrival? How many days do you intend to spend fin Dubrovnik? One night will work for Plitvice provided you arrive when you can explore the park the later afternoon evening before and the morning after. Flesh out either end and then see how many days you actually have to work with.
Also, specify what month, as that will determine if the wineries and truffles are an option.
I would probably split eight days between Zagreb, Plitvice, and Dubrovnik, with maybe one more stop en route (Plitvice to Dbv is long) but we need to know your priorities.

I arrive Zagreb airport at 10:30 am on July 6th. I will rent a car and planned on spending 1 night in zagreb.
I fly out of Dubrovnik on July 13th, 4:30 pm. So, I only have 7 days,

Is this a good time for Istria? Truffles and Wineries?
I was thinking about driving to
Rovinji on the 7th-9th stay 2 nights- daytrips to Motovun, Pula.
then drive to Plitvice on the 9th-10th (spend 1 night)
then drive south. Maybe stay in Ston ? 10th - 12th. I heard that there are wineries there too?
then go to Dubrovnik on the 12th and leave on the 13th?
I don't prefer really big and crowded cities, and I see that it it beautiful everywhere, so what is your advice.
I originally was not doing Istria at all, but a local recommended it.

Truffle hunting is a fall activity as I understand it. You may find truffle items on a menu though--I had a wonderful truffle gnocchi in Zagreb one May.

You plan is more fast-paced than what I would plan, but it is perfectly doable, provided you only wanted a night each in Zagreb and Dubrovnik. One night is sufficient for Plitvice for all but the most avid hiker, but the other places may require more time. That said, to be fair, the old town of Zagreb is pretty compact and you could likely see what you want to see in a day, and likewise, Dubrovnik is quite small. It's not my favorite place in Croatia, but even I would give it more than a day for the first visit.
I'd actually consider moving your return flight to Zagreb and just explore Istria and Plitvice based on your interests shared here. But Ston is a very pretty area to be sure-I have passed through twice but never stayed in the area. I know the area is well regarded for wine (and shellfish), but I don't have info on wineries.

Pula and Motovun can be done in the same day, but honestly, I would skip Motovun. It’s heyday has come and gone. We were there recently, last June, and it is dead. Most of the artists that Rick writes about have either aged or moved on. There are many, many houses for sale. We didn’t even stay for lunch, we drove on to Poreč to eat.
If you go, you have to park at the bottom of the town, there is a large lot, not too expensive. It is then a long walk up the hill. My husband drove all the way up and parked in a no parking zone by the cemetery. He didn’t care. Of course we got a ticket, but if you pay immediately, at the parking lot at the bottom it is half price. We paid about $11. It was worth it for him not to have to walk up the hill. I believe there is a bus, but don’t remember since we drove.
But seriously, don’t waste your time, spend it in Rovijn or Poreč.

Everyone has different travel interests and priorities. In Istria, the Venetian fishing village (now a tourist hot spot) of Rovinj was by far the highlight of the area for me - I loved it! But it's small, and personally, I would not go so far out of the way to see it or the hill towns (which I found interesting but not especially memorable - just my bias I guess). There is so much much more to see on the Dalmatian coast that I would prioritize places like Split, and island or two, or perhaps instead of islands and inland detour to Mostar in Bosnia on the way down to Dubrovnik

All of it will be busy in July no doubt. July is high season in Croatia, so everywhere anyone will recommend will be crowded.

If you don't want to stop in a big city like Split, consider a stop in the town of Sibenik a few hours north instead - much less touristy, easy to drive into off the main road, lovely town.

Last October I visited the island of Vis about two hours by ferry from Split - lovely place, probably busy like everywhere else in July but maybe less busy than Hvar or Korcula. Neat little towns and natural beauty on the island.

Istria make more sense to me combined with a trip to Slovenia.

Thanks for all the advice. I did change my flights and added 2 nights to my trip. I think I will skip Istria for another time and use my extra time on more trips south on the way to Dubrovnik.

Wise decision to cut out Istria considering your timeframe. I like Zagreb. Worth more than one night.
Driving in Croatia you will have a choice between toll roads or not part of the time. Since you are on a timeframe, use toll roads when possible. Otherwise your average speed will be maybe 40k an hour.
You don’t prefer big and crowded cities but you have to know that all the places you list, city or not, will be very crowded. To the point that you are shoulder to shoulder with others making their way at Plitvice or walking on streets of Dubrovnik. Parking may be a problem. Even pay parking will be scarce. It may take you up to an hour after you enter a town before you find parking. Traffic, unknown streets, finding the parking.

Is renting a car a good idea to efficiently see more, or should I stick with public transportation?

So now you have 10 nights?
July 6 & 7: Zagreb
July 8: Plitvice
July 9 & 10 &11:
July 12 & 13: Dubrovnik

Right? With that amount of time, you just have to make choices. If you are really interested in wine, you could consider one of the islands that produce some of Croatia’s unique wines.

If you rent a car, this is a great app to pay your parking (as long as you are in a legal parking lot). https://paydo.hr/
You can also use it to pay parking tickets (as Barbara said, half price if you pay within 24 hrs).

As Andrew said, try and go to one island. Korčula can be reached by catamaran. Or, if you have a car drive to Orebič, park the car with the app, and take a 15 minute ferry. When we were there in June 2022 the long term parking lot, across from the ferry, was being renovated. It might be finished by now. Otherwise, the other parking lot is 24 hrs and you can renew with the app. We did not have the app and my hubby took the ferry back, paid for another 24 hours, got back on the ferry and arrived in korčula all within 45 minutes.
Anyway, korčula has many wineries, located in the town of Blato. Yes, the town is called Blato, we did explain the irony. Just throwing another wrench into your planning. Sorry.

I was actually considering Korkula, thanks.