Hi, Neil. I made a stop at Predjama Castle on a drive with a guide from Ljubljana to Piran. I just looked at the outside of the castle (very impressive) and listened to my guide's story about it, which was interesting. I did not go inside. I had other things that were higher priority for me on that day -- like Škocjan Caves and the not-in-the-tour-guides but excellent Park of Military History at Pivka. Rick says the interior is not very impressive; while I like Rick's guides, I definitely don't consider his opinions gospel truth. You may want to consider that Rick has been in a lot of castles, so his perception of impressive and unimpressive may not match your perception. It sounds like his perception does not match the Trip Advisor reviews? If castles are high on your list of things to see, you may find it wonderful.
One word on caves. You have two choices. Postonja Cave is often packaged with Predjama Castle because of their close proximity. Postonja Cave comes with a train ride into the cave and is more of an attraction with a nature bent than a true natural experience. Škocjan Caves is a little further out, but very much a natural experience that requires quite a bit of walking. My original plan was to hit both caves, but I found so much in Slovenia to see that I had to narrow things down to just one. I chose Škocjan Caves and had zero regret. The Slovenians were pleased with my choice, too. I've written this elsewhere, but toward the end of the tour, I walked through an enormous cavern where it was hard not to do my best Gandalf impression, slamming an imaginary staff into the path and shouting "YOU... SHALL... NOT... PASS!"
If you haven't found it, here's my 2019 trip report on 25 days in Slovenia... https://community.ricksteves.com/travel-forum/trip-reports/25-days-in-slovenia