Thethi National Park

As in Albania?

yes.

Brendon, I don't think you should add anything else on to your 6 days in Croatia, based on your previous posts. Two days in Dubrovnik -one seeing the town and one heading to Mostar - that would be perfect. Thethi National Park is at least 250km and 5 hours of driving, just by my initial map assessment. And I haven't driven in Albania, but we did have someone drive us in Croatia and Montenegro and the roads there are slow and narrow - especially in Montenegro, and you'd have to traverse most of the entire country to get to Albania and you'd have numerous time-wasting border crossings. The places you've already picked are beautiful and you'll love your trip - but some destinations should probably wait for your next trip.

I don't think you will have enough time for Thethi - save if for a separate trip to Albania. Instead of Dubrovnik you could go to Montenegro.

We attempted a trip to Thethi a few years ago, but didnt make it. By the way i didnt plan ahead. We left early from Kotor Montenegro (a few hours closer to Thethi) and drove most of the day some dirt roads, some gravel some paved. When we got near Thethi probably another 10 or 20 kilometers the road ran out, and the only way was over what i think was a dry riverbed, in which you could only take by 4x4. I was told even by 4x4 it would be slow going and might take a few more hours even for the short trip. There were no guides available in that town at that time, so we had to turn around. I'm not sure i would recommend it. If you do decide to take the trek, you should contact a guide with a 4x4 ahead of time. The remoteness and adventure sounded cool to me but i didnt realize it would be so difficult to get there.