30 days isn’t a lot of time for so many major destinations, but that’s your call. If you are flying for some of the long legs, that might help, but you don’t see as much from a plane as from a train or bus.

I might suggest looking at a ferry from Nice to Rome, then continuing up Italy to Florence, Milan, and into Switzerland. That would save you some backtracking.

I agree. You’re trip covers a lot of territory and a lot of huge cities with lots to see and do. I’d save Barcelona for your next trip and cut Milan. Also think about what you want to see and do in Switzerland and around Nice. Both are great areas to spend several days in but not, IMO good places for a short visit as you can spend a lot of time getting to/from either without really experiencing the things that make them great visits.