We are thinking of Varenna to St Moritz, to Lucerne( Glacier Express and Mt. Titlis) to Interlaken.
Geography problem here. Varenna - Tirano - St Moritz is OK, over the Bernina Pass - very scenic.
The route from St. Moritz to Luzern is: St. Moritz - Reichenau - Chur - Zug - Luzern. That is NOT the Glacier Express route, it is a route (after Chur) around the Alps. The Glacier Express route runs west: Chur - Reichenau - Disentis/Mustér - Brig - Visp - Zermatt.
If you want to take the Glacier Express route you would be best doing Interlaken first: St. Moritz - Chur - Reichenau - Disentis/Mustér - Brig - Spiez - Interlaken. Then visit Luzern second. Train from Luzern to Paris in about 4½ hours.
You can see all this on the Swiss rail map here: https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-sts-pass-gueltigkeit.pdf
Also - standard warning. Do not stay in Interlaken, but in one of the locations past Interlaken in the Jungfrau area: Lauterbrunnen, Mürren, Wengen or Grindelwald. Local tourist site (info and hotels) here: https://jungfrauregion.swiss/en/summer/