Hey!
The most scenic routes are IMO covered by EURAIL pass and I’d recommend you to buy an EURAIL pass. The other one does only pay off if you also plan to use buses but most of the interesting bus lines so or so don’t run in December as the pass routes are having their winter break.
Scenic route: from Colmar go to Zürich. Then you’ve the opportunity to go by train to Chur, that line is already quite scenic for a main line (sit on the left side in driving direction!) - offering views of Zürichsee and Walensee with the famous Churfirsten.
From Chur you could spend a day exploring the Rhaetian railway (or get off in Landquart already - 10mins before Chur). The most interesting line is the Albula and Bernina railway which takes you first to Samedan/St. Moritz and then across the Bernina to Tirano in Italy. This is by far the best railway experience that’s possible on an EURAIL pass (of course also a bit depending on the weather). From Tirano you can catch a regional train to Milano (sit on the right side for views of Lake Como) and then from Milano you’ve frequent trains to Venice (reservation needed).
It would be also possible to put one more scenic route in your itinerary: from Basel head to Interlaken, then spend a night there (with maybe trip to Jungfrauregion - not included in EURAIL). Next morning take the train from Interlaken to Lucerne and then from there to Zurich. Then do the same as written above.