Istria Hill Country

I've only been to Groznjan and Motovun. Both are rather small (population under 1000), and I don't know that either has a local taxi at all, or at least one that would be interested in taking on a full-day job (meaning not having any daily commitments already). You might need to rustle up a driver from one of the larger places--Pula, Porec or Rovinj.

As I recall from my 2015 trip there was public-bus service to either Motovun or Groznjan; the problem for me is that I knew I wouldn't need a lot of time in either place, and there seemed to be no way to string together multiple destinations on one day--a common problem when you want to combine tiny places that have only bus service. My solution was to take the once-a-week (then on Wednesday) Inland Istria bus tour. I took the tour from Rovinj but I'm nearly certain there were also pick-ups from Pula and Porec.

Since there are two of you (at least), a private driver wouldn't be so much costlier than two seats on a tour, and you'd have a lot more flexibility, so I hope you can find something that works. Have you asked your lodgings in Rovinj and Motovun for suggestions?

we really do not want to rent a car with the hassles of driving, parking etc

I've rented cars in a lot of places in Europe (many of which I would not recommend driving); Croatia/Slovenia was by far one of the easiest for driving and best decisions for flexibility. I rented and returned the car in Zagreb and drove to Plitvice, all over Istria and up into Slovenia. Google Maps (pre-downloaded) worked perfectly. I did carry Croatian coins for parking in towns like Pula, Porec and Rovinj: I recall some meters did not take credit cards.

In that area, I was willing to accept the inconveniences for the tradeoff in flexibility. In 2017, I paid $200 for a 14 day Avis rental.

But I can certainly understand that it's not everyone's idea of relaxing, and letting someone else take the responsibility of driving has its appeal.

You should reconsider your transportation. Driving in Croatia was easy back in the Summer of 2017. We had no problems in any of the places on your list - except Zagreb because we didn't drive there. I would think you'll have more flexibility and less expense if you rent a car at the end of your 4-5 days in Zagreb and simply drive for the remainder of the trip.

I'm an Istria regular. Cars are easy there - very easy. You won't get far with a taxi as I can't really recall seeing them in these smaller towns. Do yourself a favor and rent.

I also answered your question of the Fodor's forum but in case you haven't checked back there here it is again

I understand your reluctance to bother with the hassle of renting a car but I have to say driving around the Istrian peninsula was about the easiest place we ever drove. We rented the car in Rovinj and returned it in Split (we also wanted to visit the Plitvice Lakes). If you could do a rental for just those few days it would probably greatly simplify things and we found ample parking in all the small towns and between towns was also very, very easy. In general we only rent cars when we find public transportation really not realistic but over many years we've driven in multiple places and this was the easiest.

Thank you all for such nice and informative responses. I will definitely be renting a car for my travels...from Zagreb to Rovinj to Motovun and back to Zagreb.
With regards,
Bruce