Overwhelmed

Please add some details--time of year, are flights booked? If so, where? If not, what are likely options?
Split, Dubrovnik, and Plitvice park are the three places I would consider important, but I don't believe in must-sees. If you like nature, anywhere is really good.

There aren’t any must see places, as tastes vary. Most would suggest Dubrovnik, Split and either Plitvice or an island. Others would suggest Istria plus Split.

Dubrovnik is probably the busiest place, with the tourist numbers boosted by cruise day trippers.

You could have a good holiday visiting Split, Trogir, an island (Vis or Hvar or Korcula) then the National Parks at Plitvice and Krka and miss Dubrovnik. If your flight in is to Dubrovnik, then Istria isn’t the best suggestion as it’s too far away. The islands are dead in winter - when are you visiting?

My husband and I are planning on arriving the end of September 2020.... so I think we have a lot of time to fine tune, which is why I’m posting my “overwhelmed” question now. We will be coming from Italy where we plan on visiting for 1 1/2 weeks too. We both like hiking; beaches; we don’t drink but enjoy culinary delights! Your help is sooooooooo appreciated as we want to make sure we optimize our time in this beautiful country!

That's eons away! I'd be spending time with a book (like Rough Guide and Lonely Planet, if that's okay to recommend on this site, lol). You have lots of time to tailor your trip to make it fit your interests. It should not be quite as crowded then, other than Dubrovnik. Easy for me to say after having seen it, but Dubrovnik is one of the places most mentioned as suffering from "overtourism"--and it would not be in my top five even after four visits to the country. Focus on the natural splendor of Croatia and you cannot go wrong. With a week and a half, I'd stick to a fairly confined area, but again that will depend on where you enter/leave the country.
You have many more options coming from Italy, but that also depends on where in Italy. If Venice, then Istria could be very nice in early fall (October is truffle season), and you can take a ferry over and then rent a car. You could explore hill towns and also get a few days away from it all on one of the northern islands.
If Venice, northern Croatia are not it, and without several weeks at your disposal, I would skip the other longer ferry crossings and search flight options.

1) Get the RS Eastern Europe book
2) Look at the RS Croatia recommended itinerary and other info
https://www.ricksteves.com/europe/croatia
3) Watch the RS show about Croatia
4) Surf the web

Everyone’s advice is so helpful! Thanks!

Have fun planning. We were just in Croatia. It's a great place to visit - we've been 5 times since 2011. Planning can be a lot of fun. Don't be overwhelmed.

I will give you one thought: Croatia is 3 things: the Dalmatian/Adriatic coast, the parks (Krka, Plitvice), and the interior (Zagreb, Slavonia to the East). These naturally lead to patterns of a visit.