Croatia Itinerary Oct 1-12, 2018

This is not a relaxing schedule. Once you get into October, I would be tempted to spend longer on the mainland than on the islands, as there is more to see if the weather is inclement. Plus, there is a higher chance of the Krilo catamaran being cancelled due to the weather. This happened 2 days when I was there mid September a couple of years ago.

No Plitvice?

What are you going to do with your luggage with so many early morning or early evening arrival/departures?

Many apartments on the islands have a 3 night minimum, even in the off season or you pay a premium for a shorter stay. Hvar town is very expensive. Accommodation in Stari Grad is considerably cheaper and food and drink there is about a third cheaper than Hvar town.

I would drop Brac and just have one night in Dubrovnik at the end, as you should have covered this at the start of the trip. Pity you didn’t book open jaw tickets into Dubrovnik and out of Split to avoid a day back tracking.

Cover Hvar as a day trip from Split to avoid a one night stay.

Add time at Plitvice and more time in Split to allow a visit to Trogir.

As you will have covered the destinations served by the catamaran on the way to Split, why not take the bus back to Dubrovnik, which runs more frequently than the catamaran and will give you different sights. It takes the same time.

Some of the restaurants and hotels close on the islands at the end of September. I usually use Trip Advisor rentals or Booking.com as they offer better value than Airbnb, often for the same place!

At that time of year, I would be seeking to minimise the number of catamaran trips due to the higher risk of cancellation. The ferries are more stable and will continue to operate.

This is a general comment, not just Croatia. A big advantage of staying in hotels is that they will usually store your luggage for you if you arrive before your room is ready so you can go ahead and sightsee prior to checking into your room.

Only you can know what kind of pace you like when you travel, but I would want longer stays in fewer places.
I prefer apartment rental, which you can do through airbnb or whatever portal you prefer.
I don't think at that time of year you need to book ferries/catamarans.
I hope you have great weather!

Thank you for all the incredibly helpful comments. They are indispensable as I revise our itinerary!

Geez, more days traveling that actually touring. I would choose either Hvar or Korcula. I would stay 3 nights min in each place. Why do you go to Dubrovnik 2x? I'd go once. Arrive Dubrovnik, depart Zagreb or something like that. I'd go

Dubrovnik - 3 nights
Hvar - 2 nights
Split - 3 nights
Bus to Plitvice lakes - 1 night
Tour Plitvice Lakes
Bus to Zagreb
Depart Zagreb

I've been generally happy with the Airbnbs I've visited. That's not always the case with hotels that had decent review scores. I generally pay a little less for Airbnbs too. I rented a small apartment on the Riviera (Juan les Pins) last September for under 50 euro per night. In October, I'd expect Airbnb's in Croatia to be dirt cheap.