Allow me to introduce you to the Rome2Rio website. It has some serious limitations. You should never trust the fares, travel times or frequencies it displays. But it's usually accurate about what basic transportation options exist (train, bus) and where you might need to transfer. If you keep clicking down through the website, you'll find the names of the transportation companies Rome2Rio is suggesting. Go to their websites (links are often provided) to find accurate schedules and fares.
Rome2Rio is suggesting you can put together a bus connection (via Trieste) that will get you from Venice to Ljubljana in less than 4 hours. I haven't done the digging to see whether that's accurate. It involves two different bus companies, which is a bit nervous-making. I believe the train schedule is such that it would take longer to make this trip by train, but I haven't verified that.
Quite a few folks from this forum have used GoOpti to travel between Ljubljana and Venice or other cities in the area. They all seem to have been happy with the service. As I understand it, you can either book dedicated transportation for just the two of you at your choice of time, or you can book two seats in a shared van, in which case the company chooses the departure time, and I believe you don't find out what that time will be very far in advance (maybe the previous day??). Needless to say, the first option is quite a bit more expensive than the second. I don't know whether there would be enough demand for shuttle service from Venice to Bled to make a shared transfer to the lake an option, but there's bus service between Ljubljana and Bled, and even taxi service between the two wasn't horribly expensive when I visited the area in 2015.
For the trip from Ljubljana to Rovinj there is no rail service. You're crossing a border, and international buses typically run infrequently. Rome2Rio lists only Flixbus as an option, and just once a day. The time is given as a bit over 3 hours. I traveled that route in 2015 by bus (not necessarily via Flixbus--I don't remember), and it was pleasant enough. In my experience there can be delays at border crossings between Slovenia and Croatia. I've heard they are often caused by beach traffic, so the day of the week you are traveling could be a factor there.
It will be handy to have a car for traveling between Rovinj and Split, but for what it's worth there are public buses connecting Rovinj to Pula, Porec, and the cute little coastal town of Vrsar. As of summer 2015 there was limited bus service from Rovinj to either Motovun or Groznjan (another town in the interior), but having a car would make you a more mobile and efficient traveler.
I generally tell people they can manage with just one night at Plitvice, but that's on the assumption they won't try to do any more hiking than the longest marked-out route through the park. If you want to do more than that, you may appreciate having the extra time. Before the pandemic the park was selling two-day passes as well as one-day passes, but there was an occasional report that if you stayed in one of the park hotels (rather pricey for what they are, apparently), the hotel would endorse your single-day ticket so you could use it the next day. I have no idea whether that is still going on. After multiple years of grotesque overcrowding in the park (including on the day of my visit), a new policy was put into place: You aren't guaranteed to get into the park if you haven't bought a ticket in advance. I do not know whether the number of tickets sold was reduced from the too-high (probably unlimited) level of 2015, but at least you don't have to stand in an hour-long line, as I did.
If you run out of things to do in Dubrovnik, there's bus service to Kotor that takes you around a large part of the scenic Bay of Kotor.
