Itinerary advice

Will you be able to depart immediately, or do you need to shake off jet lag before driving? Assuming 2-3 days for Ljubljana and 5-6 for Porec (from which you can visit Novigrad and places around Istria, OR, you can choose a couple bases) it will be tight but "doable" as some say.
Why Novigrad/Porec? Knowing why you chose certain places helps us advise.
Google Maps is is fairly reliable for drive times.

We arrive around 7pm in Zagreb so hopefully can get some good rest before departing the next day. Plan to stay at least a half day there. I chose Novigrad and Porec based on other peoples suggestions on this forum.

You should have plenty of time.

I would rent the car the day after you arrive unless you spend two nights in Zagreb, it is after summer holidays so things will start picking up again around September, there is no need for one in Zagreb and Bolt to city centre is still rather cheap, probably 20€. There are no hotels near the new airport terminal so stay in the city, best restaurants are in city centre as well, any restaurant on Teslina I can recommend.

Ljubljana is prettier than Zagreb as a river runs through the city centre but is much smaller and there is less to do. Make sure to look at visiting lake Bled, Postojna caves are also interesting attraction as you drive to Istra. Piran and Portoroz are the tourist attractions on small Slovenian coast.

For driving in Slovenia on motorways you will need vignette, you can buy this online before you drive there.

You should also look at Rovinj, it is the most popular destination in Croatia, though you might prefer Poreč. I am not sure of any wineries offering accommodation.

Between Istra and Zadar there are quite a few things to take a look at - Opatija is a historic Austro-Hungarian tourism town with nice parks and a promenade, Rijeka isn't really known for tourism but was European Capital of Culture a few years ago, some coastal towns and islands such as Krk, Crikvenica, Senj, Rab, Pag which you could look at, and you could easily stop for a night at one of the national parks Plitvice, Sjeverni Velebit, or Paklenica. Coastal roads between Labin and Opatija and Rijeka and Rovanjska are particularly beautiful so have a look and see what suits you.

Wonderful! Thank you so much for the input!