Slovenia part of our trip -- advice please

It’s feasible, my only question is when are you planning to see Lake Bohinj? I don’t see it on the itinerary.

Regarding driving in the mountains, it’s unlikely there will be snow on Vrsic Pass in the last week of May, so no need to worry. In the off chance that there is snow, there are other ways of crossing into the Soca Valley, one of which is over another, lower but only slightly less scenic mountain pass, Predel.

In case you were planning on driving the Mangart Road while in the Soca Valley, that one will probably be closed until mid-June, but there are so many other sights in the valley that I wouldn’t worry about it.

Seeing I only have 5 nights to divide up my itinerary in Slovenia. Want to see the caves like Skocjan and Predjama castle , Ljubljana, Lake Bled and Bohinj, Julian pass and soca Valley.
Figure do caves and castle on a Thursday on way from Rovinj in Istria, to Ljubljana.

Option 1: Then spend 3 nights (Thursday, Friday, Saturday) in Ljubljana, Will need to figure out place to stay here, might try an apartment but need parking as will have car. Has anyone used Air bb here. Very strict cancellation policy--- usually half of rental if cancel after original booking. Any suggestions for place to stay in Ljubljana.

Spend Sunday and Monday night in Lake Bed area see Lake Bled and Lake Bohnj. Do Julian Pass Monday but return to Lake Bled for night. Tuesday explore Lake Bihinj and drive to Zagreb This leaves options more open if weather poor.

Option 2: Thursday and Friday night for Ljubljana. Saturday and Sunday night for Lake Bled and Lake Bohinj. Monday drive the Pass and stay in Soca Valley area Monday night and drive Tuesday to Zagreb exploring on the way. Either way not going to explore much Soca Valley are we?

Having read how much people liked Ljubljana I have thought that 2 full days would be desirable.
Oh to have another night/day for Slovenia.

We were in Slovenia in September and spent 3 nights in Ljubljana. The city is certainly worth 2 full days at least. We could happily have stayed longer just wandering around. We stayed in the Grand Hotel Union just yards from Preseren Square. The hotel was very comfortable and the breakfast was excellent. It is attached to the Grand Hotel Executive which has a parking garage.

We also visited Skocjan, which is about 10 minutes drive from the motorway. The excellent tour lasts about an hour and you may have some waiting time before the tour starts, as well as a short walk to the cave entrance. Predjama Castle, which we didn't visit on this occasion, is near Postojna, about 30 km from Skocjan. If you're visiting the castle anyway, it may be more convenient to do the Postojna caves instead of Skocjan.

The Grand Hotel Union in Ljubljana is usually expensive, but it can go on sale near the last minute if occupancy is low. I stayed there one night a few years ago - not only is the location perfect, the view from my room (the castle, the Franciscan Church, etc.) was breathtaking - the only place I've ever stayed in Ljubljana with a view, actually and worth it for a splurge for sure.

In five days you can see major sites you mentioned. Three nights in Ljubljana are fine, but I would visit Bled as day trip from Ljubljana and not at weekend as can be busy. Hotels are many in Ljubljana with parking, look at booking.com as is easy to cancel or change bookings.
You can do Vrsic Pass and Soca Valley Sunday and Monday. Lots to do in area in two days and then move to Zagreb. Late May no snow problem, so don't worry. On route to Zagreb from Soca Valley you can go more scenic way towards Idrija, Cerknica, Bloke, Kocevje, Vinica, Karlovac to Zagreb as you have all day and you will see some other nice parts of Slovenia.
Buy vignette for Slovenian motorways before you enter Slovenia.

Going to be hard to avoid Lake Bled or Bohinj on a weekend. Hoping May not as busy as later in season. Ljubljana was suggested for a food market on Fridays. So was going to go there first but either direction would place Lake Bled on a weekend if doing in some kind of circular route unless I have this wrong.

Late May is perfect for Slovenia. Wildflower festival in Lake Bohinj and the best walking/hiking trails. I find Lake Bohinj about 100x better than Bled. Bled is very touristy and tatty now. Was there 10 years ago and returned last year. Didn’t recognize the place.

Any suggestions where to stay in Lake Bohinj as may be more appealing to stay. Are you able to buy driving vignette for Slovenia at gas stations or other in Croatia before you cross the border. Certainly would not want a fine.

Bohinj is more "back to nature" than Bled. If you prefer that, then seek out a place on say Booking.com . I didn't care for Bled the town, but it is a convenient base, and the lake is beautiful. I would never urge anyone to skip Lake Bled, an iconic sight in Slovenia. It is more touristy than Bohinj for sure, but not awful, and it is also more practical in the sense that it's easier to find restaurants etc. Given the short time you have, I wouldn't get so hung up on where to stay. I'd pick the most practical place.

I was last in Bled back in mid-May. It was a slightly rainy day, but I still enjoyed walking around the lake which I'd seen before. Very few tourists, but some were taking pletna boats out to the island. You need not spend a whole day at Lake Bled but it's worth a brief stop at least.

Visited Bled last Sept, nice but, pretty bland. I plan on returning and spending more time in Bohinj, which I understand is more naturally striking and varied, as they also have a lift-area for skiing in the winter.

Definitely stay in Kobarid, if anything reserve a dinner and possible stay overnight at Hisa Franko, Ljubljana's best restaurant and home to the world's best female chef Ana Ros. She was featured in Netflix Chef's Table. One of the better meals I've ever eaten and definitely got a better idea of Slovene cuisine.

Kobarid also has an impressive WWI museum, talking about the battles that surrounded the area, which also served as the setting for Hemingway's Farewell to Arms.

I presume you will drive over Vrsic Pass from north to South, I.e., from Kranjska Gora, if you do decide to drive it. A part of Slovenia that is quite beautiful but off the main tourist track is the road from Bled to Kranjska Gora via the Radnova Valley. It sort of parallels the main route but sees little traffic. It is now fully paved (partially gravel just a few years ago). I’ve bicycled it twice now and it is a marvelous, albeit “slow” and (no center line) narrow road (as in a U.S. National Park type of slow road where you can look around and stop here and there). It is through forest, sometimes following a small river, and passes a few farms, a memorial to citizens murdered by Nazis, and here and there views up to the mountains. It starts a few miles outside Bled and ends a few miles short of Kranjska Gora, but a road map & GPS should make it a snap to find. I would think a casual drive might take you about an hour or so for the entire drive from Bled to Kranjska Gora (where there are lots of accommodations).