8 Days in Croatia - I need help with the last 4!

You could use some time in Zagreb, and Zadar is also nice. Neither helps you get to Mostar, but it's probably at least a 10-hour round-trip from Plitvice Lakes National Park, a long way to go for such a quick visit. In any case, with only two nights available, if you really want to see Mostar, that's where you should go, not to some other city from which you can make a day-trip to Mostar. Two nights just gives you one day plus a few hours of sightseeing time.

I have no recent experience with Croatian islands, but it has been mentioned on this forum that a lot of island hotels have 3-night minimums, and you're traveling during peak season. The Rovinj lodging situation was really tight when I stayed there in 2015.

If you're contemplating taking the car on a ferry, be sure your rental agreement allows you to do so. Ditto for traveling into Bosnia.

I'm wondering whether you're allowing yourself any time to see Ljubljana, since you say you plan to take day-trips every day.

Thanks Acraven, you may be correct about Mostar - I think we have to decide between staying on an island (we can use airbnb) or giving that a miss and stay on the mainland for 4 nights and visit Mostar). I was hoping we could do a decent drive from the lakes to somewhere like Markarska, get to Korkula perhaps then the last 2 nights in Markaska. Any views on Markaska as a place to stay?
Going to Bosnia isn't an issue but the ferry ma be.

Look into Medjugorje. It is the most peaceful place in all the world and lots of history :)

Given that you must return to Zagreb, I would stick north, and there are some fabulous islands in the north. Not as famous but just as ruggedly beautiful--look at Cres, Losinj, and Krk. They are also hardly undiscovered, and the towns will be lively in July/August.
Or I would more exhaustively explore Istria while you are there. I would not drive all the way to Mostar but instead keep that and southern Dalmatia for the future trip.

Thanks for those suggestions - Mostar is off the radar and now I guess we have to decide on either the islands (I suppose we could base ourselves on Krk and still see Losinj and Cres?) or stay on the mainland for 4 nights. If it was the latter, what would you prefer from the following:
Zadar, Sibenik, Primostenor Trogir?

Thanks for those suggestions - Mostar is off the radar and now I guess
we have to decide on either the islands (I suppose we could base
ourselves on Krk and still see Losinj and Cres?) or stay on the
mainland for 4 nights. If it was the latter, what would you prefer
from the following: Zadar, Sibenik, Primostenor Trogir?

Look at drive times to see how much of an area you wish to cover.

See ferry schedules at https://www.jadrolinija.hr/ for Cres, Losinj (those two are connected by a bridge) and Krk (Krk also connected to mainland by a bridge).

I would start in Rovinj 2 nights
Zadar 1 night
Split 3 nights - side trip to Trogir
Split is a wonderful town, with a gritty feel. It's a place where people actually work, which is not true for all towns.
Go to Plitvice on the way back from Split to Zagreb

If you do not want to go to Split (an odd decision - we've been there 2x - lots to see and do), go to Sibenik. It's small, but fun, and has the world's craziest fountain.

Thank you everyone, your suggestions have been very helpful. I think we will stay in the north and extend our stay in Rovinj then spend two nights on Krk with a visit to the lakes at the beginning or in the middle.

I will add at least one island on it. From Korcula, you can visit one more national park Mljet. Mljet and Plitvice are 2 things must see.