I am a strong advocate of the Bernina Pass route from St. Moritz to Tirano and then Varenna, but not a fan of the Glacier Express. You would basically spend all day (8 hours) on the train to reach St. Moritz, which as a hiker myself, I would consider a lost day. You can get to St. Moritz slightly faster by another route, via the deep tunnel and Zurich, but that is definitely not scenic!
Since you know you like Lauterbrunnen and would like to return, I suggest you do that, and then spend 1-2 nights at another place on the way to Milan. I can suggest two good candidates:
One is to go from Lauterbrunnen to Kandersteg, on the line down to Brig, where you pick up the train to Milan. Kandersteg itself is a nice scenic village, but I suggest you take the gondola up to the lake and overnight at the mountain inn there.
https://www.oeschinensee.ch/en/
We have enjoyed a number of stays at this hotel, and there are some outstanding hikes from the lake.
The other possibility would be a village high above the Rhone Valley just east of Brig. Our favorite is Bettmeralp. You reach it by large cablecar ( like the one up to Mürren from Lauterbrunnen) starting from Betten, about 12 minutes from Brig.
https://www.aletscharena.ch/nature-en/the-great-aletsch-glacier/
Scroll through the photos to find summer scenes showing the Glacier. You can hike along the ridgetop above it for a mile or more if you like, or take a trail farther to a small lake and lunch hut, passing very close to the Glacier on the way.
Note that between Zermatt and Spiez ( where you catch the train going west to Interlaken to meet the Lauterbrunnen train) the main train line goes via the long and deep Lôtschberg Basistunnel. You can avoid that by continuing on the train from Zermatt past Visp to Brig, changing there to a train that goes through Kandersteg on the way to Spiez. There is a short tunnel on this route as well, but not as long as the Basistunnel. The same advice applies to your southbound trip,to Italy: between Spiez and Brig, take the slower train via Kandersteg to avoid that tunnel and enjoy more scenery.
