I agree with the other posters, time is your constraint here. Your original itinerary is good for the highlights of Slovenia but it’s too short for Dalmatia, and transportation is an issue given the range of your itinerary and you leaving out of Venice. With this plan you need to be on the move all the time and I feel you will spend more time than expected planning/figuring out the multiple transportation modes.
To make the most of this type of trip I would at a minimum flyout of Zagreb instead of Venice, so you could have a single car rental from Split to the end of your trip (or even for the whole trip — Korcula and Hvar are not small, it’s not pointless to have a car there, the Peljesac peninsula is pretty, etc). That way your itinerary could be more efficient, going for example from Plitvice to Piran, setting you up for Vrsic Pass through the Vipava valley, or Postojna on the way to Ljubljana, etc. That would also enable you to properly do the beautiful Adriatic Road in Dalmatia and the Kvarner.
To make the most of this trip, I would consider adding 4 more days. Or with the same total time, you could stick to Croatia, with a side trip to Mostar or Kotor instead of Slovenia. If you keep the Dubrovnik-Venice plan, you need to make a few more difficult choices. In my mind, the main risk here is that you’ll wish you stayed a few more days in Croatia but you gotta get on to the next stops, and in the last few days of the trip the wow factor is not there as much (although Vrsic/Vintgar are different from the rest).
It’s not easy to do Croatian islands on a day trip when you are already on an island, most popular day trips begin on the mainland (Dubrovnik to the Elaphite islands, Zadar/Murter to Kornati, Split to Blue Cave etc). On multiple trips we have been to Korcula, Krk, Cres and Losinj, they are all beautiful and a little different but you don’t have much time, just pick one. You will have some idea of the scenery just by taking the boat anyway. Keep in mind that without a car, visiting another island will basically mean visiting the main town on that island and nothing more.
Regarding Vintgar Gorge: it is a very different experience from the Plitvice Lakes, I don’t think there is much overlap. We have done both multiple times and they are always great, but you need to plan well and get there very early to beat the crowds and get the best light.
Regarding Vrsic Pass: the drive and views are the highlights, the Russian Chapel is pretty but it is a quick stop. There are countless hiking possibilities around Triglav if that’s your thing. The Soca River valley south of the pass is beautiful, whitewater rafting is a big thing there but there are also many opportunities to stop and take in the scenery. Further south in Kobarid there is an interesting WWI museum, and if you’re into that type of splurge you could consider booking dinner at Hisa Franko (we went before they got their Michelin stars and had an amazing experience, the price was OK back then… now, not so sure!). The rest of the valley has pretty villages that are a blend of Slavic Europe and Italy (Most na Soci, Kanal), and that part of Slovenia has two very underrated wine regions (Brda and Vipava) but you don’t have much time.