Lucern to Thur early train to catch Bernina Train

You could also take the 6:10 with a train change at Zurich Hbf. It is a 10 minute change in Chur, but it is cross platform from track 9 to track 10.

https://www.sbb.ch/en/buying/pages/fahrplan/fahrplan.xhtml

6:10 Departing Luzern arrive Zurich HB 6:56
7:07 Zurich arrive Chur 8:22
8:32 Chur to Tirano (Bernina Express) 12:45

Thank you, I was worried 10 min might be cutting it too close??

Would it be smarter to go to Chur the night before, just staying in Lucerne one night?

The answer to your question may lie in another question? Do you want to take breakfast at a hotel or on the train? If you would like to enjoy a hotel breakfast, pre-position yourself for the 8:33 train departing Chur.

An alternate would be to spend the afternoon and night in a mountain resort such as Davos. Take the 8:31 train from Davos Platz to Filisur connection to the BEX at 9:33 arriving Tirano 12:45.

More than 5 minutes in Switzerland is a comfortable connection.

Edgar's point about breakfast goes deeper. You pay for a hotel room in Luzern. And then get up in the middle of the night. The rate in Switzerland usually includes breakfast, but you will be too early for it. So half of what you paid for the night is wasted. and you'll be so tired you'll fall asleep on the Bernina Express!

Chur has a nice old centre and is worth at least an evening stroll. Find somewhere in the old town or the area between there and the station and stay there - furthest walk would be 10-15 minutes, most much closer. You may find that Chur is comparatively cheaper for hotels than Luzern. http://www.churtourismus.ch/en/home.html

And don't just rush for the fastest train, the journey from Luzern can be quite stunning with a little thought. Take a train branded as Voralpen Express from Luzern mid-late afternoon (so you still have most of your day there). They go hourly. For example at 16:39, this runs via Arth-Goldau and you get views of the lake and the Rigi mountain. After Arth-Goldau there is a steep climb and 40 minutes of near continuous views of rural Swiss farmland with rolling hills. And then it drops steeply down to Lake Zurich and the station at Pfäffikon SZ. Change trains here (you have about 40 minutes so no panics). Then 17:41 Regional Express train to Chur arrive 18:43. The journey planner suggests stay on at Pfäffikon SZ and change at Uznach and Sargans to save some time, but that has a 3 minute connection and I don't think you'd like that!

Thank you all for your advice! I think staying in Chur one night, taking those routes sounds ideal! I appreciate all your time & help!

We returned from Tirano to Klosters on the shorter route going through the Vereina Tunnel. Not much scenery traveling thorough the 19 km tunnel. The connection was three minutes but absent of any drama. Train from Tirano arrived on one side of the platform. Train to Klosters arrived on the other side of the platform and we hopped on without any excitement. Three minute connections are very doable in Switzerland if you don't have to cross under or over tracks. I believe the term is Swiss precision.