Slovenia/Croatia options from Rome

I am not at all sure the rail pass is going to be financially worthwhile for you in Slovenia and Croatia. Perhaps you'll get value for it elsewhere, but I'd take a look at the cost of a shorter pass that wouldn't cover your time in Slovenia and Croatia to see whether it would make more sense to just buy individual tickets in those two countries. As I think you're beginning to realize, there aren't a lot of trains in there, so you won't be running up a lot of miles on the rails. The trains that do exist aren't very fast, so buses (usually cheaper) may sometimes be an attractive option, depending on their schedules. I just question whether individual tickets will cost as much as the pro-rated daily cost of a rail pass.

I don't buy first-class rail tickets and don't know to what extent there are first-class carriages on the trains in Slovenia and Croatia. The Seat61 website may have information that helps you.

I believe there aren't a lot of public-transportation links between Slovenia and Croatia, so I'll point out that Ljubljana and Zagreb are connected by both trains and buses. A lot of people don't plan to visit Zagreb, but it's a very attractive city with a large historic area. I'd say it has a sort of Central European look, rather than a Mediterranean look.

Lake Bled is beautiful, but it is tiny. You can walk all the way around it in two or three hours. To me, it's perfectly visitable from charming Ljubljana. That's what I'd do if I didn't plan to press on to Lake Bohinj or beyond. There is bus service between Lubljana and Bled. When I was in the area in 2015, there was always a taxi hanging out at the Bled bus station just before the bus for Ljubljana was due in. At that time, you paid 7 euros for a ride in the van back to Ljubljana, which saved a bit of time. I never observed anyone offering a similar service to Bled.

Plitvice Lakes National Park is a treasure, but it is horrendously swamped during day-tripping hours. I recommend it only if you are able to spend a night in the area and see the park late on your arrival day and/or early the next morning. It seems that if you stay in one of the park hotels (rather mediocre, and I assume sort of overpriced), you don't have to pay for a two-day ticket. There is bus service from Zagreb to Plitvice and from Plitvice to Split.

I don't know about bus service from Plitvice over to the Istrian Peninsula. It looks challenging. Istria is a bit tough without a car. This might be a place to consider using a private transfer. I have no idea whether GoOpti works in that area, and if it does, I fear a less expensive, shared transfer will not be an option. You could also check daytrip.com for a private transfer.

Most of Istria is Croatian; only a sliver is Slovenian. If you want to move around the peninsula to see multiple towns/villages, you need to figure out which places are important to you and choose a base in the same country. The only Slovenian-Istrian place I've stayed is Piran, and that was very briefly and 30+ years ago. I haven't been to Portoroz or Koper. I'm more familiar with Croatian Istria, having been there in 2015. It's easy enough to get a bus to Porec (colorful), Pula (but its Roman ruins may pale after Italy) and cute little Vrsar. I took a bus tour (used to be once a week on Wednesdays, but I don't know that it runs in May) that got me into inland Istria, including Motovun and Groznjan; it also stopped at a winery. As is so often the case, it's difficult to get to more than one small town a day when you are bus-dependent, but you can probably hop off the bus in Vrsar on the way from Porec to Rovinj.

You didn't mention the caves in Slovenia, but both the Postojna (more touristy) and Skocjan (more natural) caves are accessible by public bus, though you'll have to walk from the highway to get to Skocjan.

I know you have said you would prefer not to rent a car, but for some parts of your
itinerary, I think you may want/need to consider it.

As noted, trains are not frequent and don't have that much coverage. The Flixbus
approach will work for going between cities, but not as good for places like Plitvice.
You may end up with unwieldy schedules and one mistake getting off at the wrong
stop, or waiting at the wrong stop, will have large consequences.

Here is one theoretical option:

• fly from Rome to Split
• see Split if you want, but rent car, drive to Plitvice, see Plitvice
• drive to Zagreb, drop off car, see Zagreb
• bus to Ljubljana
• see Ljubljana, then rent a car to see Bled, Rovinj, Julian Alps, caves, etc.
• drop off car in Ljubljana, take bus to VCE for flight home.

Driving away from the cities in Croatia and Slovenia is not a major issue. You just need
to make sure you document the condition of the car when you get and that you're
covered for tolls however they are paid in the country you are driving in.