I was in Croatia three years ago, with another company and visited many of the places on the GCT itinerary.
re: Dubrovnik - You will receive a nice overview of Dubrovnik on Day 3 of your trip. This is beautiful walking city and the walk on the medieval walls (takes about 2 -3 hours) is magnificent and not-to-be-missed.
If you don’t mind giving up a day on your own in Dubrovnik -- these are my thoughts on the optionals:
Montenegro: this is a beautiful drive along the coast and much of the interior of Montenegro is hilly. There is a walled city which is a smaller version of Dubrovnik with the same kind of architecture, shops, cafes and restaurants. There are lots of vendors at the car and bus parks selling embroidered tablecloths.
Mostar: is a UNESCO site and I felt it was much more historically and architecturally interesting than Montenegro. If you go, wear shoes with very thick soles as the cobbelstones are hard on the feet!
If I could do only one of the day trips from Dubrovnik, it would be Mostar.
Istran Peninsula with Rovinj and Pula: Rovinj is a nice city by the water. But you will enjoy visiting Pula if you like Roman ruins. I think it is 6 of one; half-dozen of the other on taking this day trip
Taste of Medieval Slovenia tour: one of the most pleasant things to do at Lake Bled on a warm, sunny day is to walk around the lake. There is a paved walkway all the way around and it is about 3 1/2 miles. There are numerous small restaurants and you can also stop at Marshal Tito’s former summer home (now small, posh hotel - go upstairs to see the murals in the dining room) for coffee or tea. You can take a 5 min. boat ride to the church in the middle of the Lake, as well as visit the Castle overlooking the lake. I would stay and enjoy the lake unless you are particularly interested in the architecture offered on the optional tour.
About the Valomar Lacrona Hotel in Dubrovnik-- not all rooms have a sea view which is a shame. I did two trips in Croatia back-to-back and the second trip stayed at this hotel. It is a long walk or a bus (stops in front of the hotel) or taxi-ride to the walled city I upgraded to a sea-view room and, as I recall it was about 20 euro extra a night. The room was exactly the same size and decor as my original room overlooking the parking lot, but I really enjoyed sitting on the large balcony watching ships come in to port and the sunsets. Let them show you a few rooms. The higher up the better.