I hope this travel-planning is a pleasant and relaxing diversion from your bar study; I recommend that. I took the bar while working full time as a law clerk (state bar membership was not required by the federal courts at that time), and I believe that some kind of diversion from the grind of bar review courses is a good thing.
I am glad to see that you decided to head straight to Luzern and spend 2 nights. With your interest in Hans Urs von Balthasar that makes sense. I read a bit about him and his connections to Luzern, particularly the churches where he attended Mass, and the early school he attended. With a full day there, you will have time to see both the Gothic Franciscan church (St. Maria) and the baroque Jesuit Church he found so beautiful. And should you wish to visit the Benedictine Abbey where he first attended school, Engelberg is a short 43-minute train ride away. It is a lovely mountain village, now a ski resort; your visit there could include nice scenery and a quiet lunch at a local café.
Luzern is quite compact and its position right on the river Reuss is very pleasant. The two churches, the train station, and numerous nice hotels are all quite close to one another on the same side of the river. Cross the pedestrian bridge (Kapellebrucke) to reach the Old Town and more hotel options, some right on the river.
I see some have suggested you take the Bernina Pass route to reach Varenna on Lake Como. This is possible, but it will add so much extra time I recommend breaking the journey into to days with an overnight stop on the way. You need to get to Tirano, well to the east on the Swiss-Italy border for the train to Varenna, and from Luzern that is not easy.
Here is a rail map of Switzerland to see the routes. Varenna is not marked on the map, but it is directly across from Menaggio on Lake Como.
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelpass.pdf?sfvrsn=7788603a_56
From Luzern to Tirano, the fastest rout (5.5 hours) actually takes you south via the long and deep Gotthard Basistunnel to Lugano, then to Milan or Lecco and then to Tirano—-passing right by Varenna in the way! And you miss the Swiss part of the Bernina route entirely.
The slower way, using the entire Bernina Pass route from Chur, takes you from Luzern back to Zurich and then Chur to reach the full Bernina route, passing through Swiss villages such as St. Moritz, Samedan, and Pontresina before heading over the pass itself and down to Tirano. It is a scenic journey, but well out of your way.
My suggestion, if you do have an extra night to spare before reaching Varenna, would be the Gotthard Panorama Express from Luzern to Lugano. This is a scenic trip by boat from Luzern down the lake to Flüelen, where you board a train over the traditional Gotthard Pass route to Lugano, a lakeside town in the Ticino (Italian-speaking region of Switzerland.)
https://www.myswitzerland.com/en-us/experiences/gotthard-panorama-express/
This route runs only once a day, with the boat departing Luzern at 11:12 am. You arrive in Lugano at 16:41. This would be the place to overnight. I have long wanted to stay at this little hotel right on the lake, actually in Gandria (a short taxi ride from the Lugano train station). The website appears to not be up to date, but I have seen reviews from April of this year, so I believe they are open.
https://www.elvezialago.ch/
Of course there are nice hotels in the town of Lugano itself as well. And possible this lakeside town would meet your wishes for a lakeside town, and you could spend 2 nights here instead of heading to Varenna.