Luzern to Bernina Express to Milan

Swiss trains set the standard for on-time performance. If you are really nervous, you could take the 4:52 am train to Thalwil, getting there at 5:41 am. You will have to take the underpassage to get between platforms at Thalwil.

The regional trains are great, and the windows open, so one can take photos without window glare. And since there are no seat reservations on the regional trains, and I have found many open seats, one can bounce from side to side of the train, to catch vistas on both sides.
The Swiss train system is world class. Connections are efficient. Download the SBB app, and know your arrivial / departure platform numbers. carry-on luggage is recommended, as often lifts are hard to find, so you may be navigating steps. Be ready when the train rolls into the station to immediately get off the train. Enjoy this beautiful ride!

Thank you so much! It helps knowing that others don't think that my plan is beyond crazy. Another question - do either of the trains (regional or Bernina) have any sort of "guide"? I'm thinking some sort of narrative given through the trains as you pass landmarks and such. I might be interested in paying for the panoramic trains if that is offered on those cars but not the regional cars.

Bernina Express has headsets with multi-lingual descriptions of the sights as you pass by them.
From the Bernina Express brochure:

Your digital travel companion The InfoT(r)ainment provides you with thrilling information on the UNESCO World Heritage. Listen to interesting stories using the audio guide or follow the route live on the interactive map. Just register and you’re ready to go – listen and enjoy the view. Use the InfoT(r)ainment free of charge on your own smart phone, tablet or laptop using our Wi Fi – you will not need any mobile data. You need headphones to use the InfoT(r)ainment. These are available at the Railbar on board the Bernina Express for CHF 2.00.

Everything else is just a train to get you from point A to B. The regional trains do have windows that open for better photos, and are much less crowded so you can move from side to side as the scenery changes.

The RS Swiss guide book has an informative section on the Bernina Express landmarks. There are many u-tube videos also. If you can find the Smithsonian TV Series of "Mighty Trains" the Bernina Express segment is quite informative on the building and maintenance of the route. I found that having studied the route beforehand, I had a greater appreciation of the incredible skills it took to build the route in the early 1900's , all without computer -assisted math.
We rode the route twice in 2021, coming out of COVID, and couldn't cross into Italy. I found it fascinating each way, different sunlight, views, and knowing what was coming up. You have a treat in store!

PS The regional trains offer more trains and a more flexible schedule. Also, every seat is taken on the panoramic trains, so you won't be able to jump around. You'll have glare from the window glass, and some have reported being very hot in the summer as the sun hits the glass. I believe there are open windows between cars, so you could take pix there, if too many other folks didn't have the same idea.
Safe Travels!

1a - yes

1b - no

2a - I never have so to me, no

2b - you could do either as long as the timings work. With regionals you may change trains a couple of times - free leg exercise and lungfuls of fresh mountain air included

2c - if you are next to them you won't notice and they are plenty big enough. What you will notice is because they are not curved you won't get lots of distortion or reflections If you are in an aisle seat for a while you can look out both sides without lots of folks in the way.

2d - yes, the Swiss are proud of their trains. Some bozo may come along with a spray can, for either type of train, in the middle of the night. On the fancy named trains all the seats are full so you can't change to an unaffected seat. Not so on the unreserved regional - if you don't like the seat you are in, move to another. Or scoot across to the other side and look out there for a while.

No guides on the normal train. No tilting glasses either. Many fewer flashes reflecting off the windows because locals on the train aren't taking pictures, and the folks taking photos through open windows, or even with closed will possibly be more informed about the uselessness of flash taking pictures of distant scenes.

My experience mostly repeats what others have shared. On the regional train, for most of the trip from Tirano to Ponsetrina, there were only 5 other people total in my rail car. We all freely moved from side to side for whatever view we wanted at any time. We also had a couple windows where no one was seated fully op for taking clear photos while not having too much cold air where anyone was sitting (mid October). I opted for the flexibility of the regional train especially because I was concerned about the weather. There was a map at my seat shwoing the names of all the stops. I also had the RS Switerland book which as already mentioned had some nice details about the route and what to look for at points along the way.