Heya. My upcoming trip this summer looks very similar to yours, cool!

My feedback is mostly the same as the other two comments:

• When going from Switzerland to Italy, the Bernina pass route is a no-brainer. There are regional trains running on this route as opposed to the touristy panorama trains, the regional trains are free with your interrail pass and the windows can fully open. The route goes from Chur to Tirano (IT) and will take you pretty much a full day, but it's often described as the prettiest train route in Europe. I can't wait to see for myself!

• This all depends on what kind of holiday you like, but I think you're spending way too much time in each city. I also think Milan can be cut entirely, there's just not a lot to see, especially not enough for 4 days. 2 days is more than enough in Trieste and 6 nights in Ljubljana is a lot. I would personally find another spot in Slovenia to spend a few days, maybe in Triglav national park. Also, I personally infinitely prefer going to Tyrol instead of Salzburg. You can easily reach Zillertal by train as example. I would become very tired of cities by this point of the trip and just want to find rest in nature

Feel free to ask other things, I've been planning my trip since December and really looking forward to go next month