Count up how many days you will be travelling. It looks like 5 or 6 days to me? So you’ll need either a Global Pass 5 days in 1 month, or Global Pass 7 days in 1 month. (There is no 6 day option)
You don’t need a reservation on the Bernina Express unless you travel in the panoramic cars. The regular cars, and the regional trains on the same route, all have very big windows and are of good quality.
Good advice about the Glacier Express and how to reserve here:
https://www.seat61.com/trains-and-routes/glacier-express.htm#how-to-book-if-you-have-a-railpass
Again, you can travel on regional trains along the same route if you prefer not to pay the reservation fee. Expect it to be busier than usual around New Year
You’ll also need a reservation on the Milan to Rome train; these are very easy to get, though again, around New Year you might want to book ahead of time.