My first trip to Switzerland, we were on some very crowded trains in 2nd class. So, I always travel in 1st class now on my longer train rides, especially when travelling with luggage. I prefer more room and less crowds. To me, the extra expense is worth the money.
As far as which side of the train, my sister and I sat on the side with the single seats facing each other across a table. It seemed as if we were looking out the windows on the other side of the train. But, I didn’t find any part of this train ride to be more scenic than anything I have seen in Switzerland. We try to pick seats near the luggage rack and not too far from the bathrooms.
Since you plan to leave so early, I don’t think you’ll find any grocery stores or bakeries open before you leave. The Spar grocery store in the train station opens at 6. So, plan to get some snacks the night before.
Also, but sure to select a time with the shortest number of changes. I see a 5:56 train that gets you to Lauterbrunnen in 7 hours and 59 minutes.