Need help on exploring Croatia in October. Recommendations for 'back door' experiences, best travel agencies for help, and all around great information. Totally flexible. Prefer travel from Chicago to Split. Slavonia, Mali Ston oysters, Karlovac, Istria, truffle hunting - if precipitation cooperates, Pula as travel destinations to begin my travel homework. Rick's 2020 Croatia/Slovenia guide book helpful, out of date, or other travel books you've discovered with up to date 2022 travel info. Thanks
Can you be more specific about your interests? Sounds like you want to get off the "touristy" path. Most travel agents are going to direct you to touristy spots like Dubrovnik, Hvar, etc. Actually, Dubrovnik may be touristy (especially during the days) but it is really unique and well worth seeing. Before AirBnB, Croatia had (maybe still has) a long tradition of Sobe (short-term rentals - rooms, private spaces) as an alternative to hotels.
Pula was disappointing to me - I had already seen Roman ruins elsewhere, so I did not find the town that exciting. I loved little Rovinj, though. I didn't go truffle hunting in Istria, but it shouldn't be hard to find info on doing that.
National parks? Plitvice Lakes National Park is a touristy national park not far from Karlovac, but it's really stunning and unique - dozens of water falls and two big lakes. Go in the mornings to avoid big tourist crowds, which may still be there in October. My first Croatia trip was early October, and the park was showing some fall colors and really gorgeous.
Read back in this group for past trip reports and threads about travel in Croatia - there should be a lot. I don't think your itinerary will be much different post-Covid. Croatia has not had many travel restrictions even during the pandemic.
Rough Guide, DK, and Fodors have new editions, which you can supplement with web research. The country is super easy to travel in. Public transport is good, roads are good (you may want to drive for Slavonia and some national parks), and well before airbnb was a thing, there was a well established system of private rentals throughout the country. Tourist agencies are in general well staffed. Even for Slavonia you can find info online. For example, see https://www.chasingthedonkey.com/slavonia-croatia-travel-blog/
I don't think you need to search anywhere other than Kayak/Google for airfare (then I book direct with airlines), but if you think you need extra help, I purchased tickets from this agency once: https://www.croatiatravel.com/
Unless you have at least a few weeks, I would consider sticking north--October is a good time for Istria in particular. In Istria you have Ucka nature park. You only mentioned one thing in the south, and I'm sure you can get shellfish in Istria. Zagreb is often the best for flights, so you could do a loop from there, taking in Istria, across to Plitvice, then Slavonia and back to Zagreb.