<<"I wasn't sure if 2 days in Interlaken might be too long - my thoughts were around having one day to do a Jungfraujoch tour depending of courseon the weather. I keep trying to focus on not trying to see everything and actually not seeing anything but there is just so much to see!!">>
I can see that you have been planning carefully and thoughtfully. You have a sound approach and a good start. You are right about leaving time to allow for weather conditions at the Jungfrau.
If i were doing it, I would stay further in the valley, as noted before.
Here is something to consider. Bern is a short train ride from Luzern. Exactly one hour if you take the trains that run at xx00. An hour and 21 to 27 minutes if you take slower trains at xx04, 0034 or 0036 ( the latter is a local.)
Bern to Zürich is 56 minutes twice an hour at xx02 and xx32 plus others.
Zürich to Luzern is 46 minutes to one hour.
Bern to Interlaken Ost is 53 to 54 minutes.
Interlaken Ost to Lauterbrunnen is 21 minutes + further time to Wengen or Mürren; Interlaken to Grindelwald is 35 minutes .
Luzern to Interlaken is one hour 50 minutes.
Thus, for me an alternate progression would be Zürich to Luzern to Bern to Interlaken to Montreux. And, with those relatively short train rides, you could visit Bern from one of the other cities, and/or Luzern from Zürich Time lost on the enjoyable train rides would be partly made up by avoiding the time consumed in checking in and out of hotels.
For me there is no single optimum among those choices. Depends onyou, but there is at least a possibility to reduce the number ofdifferent hotels.
You can also consider Zürich Flughafen to be your airport for Luzern. Twice per hour - 1 hour two minutes direct; or one hour 12 minutes change in Zürich HB. The change is not too bad -you get of at track 16 and board at track 5, both on the main track level. There is a "mid-track" tunnel, so you don't have to trek all the way to the railhead. Here is a map from the SBB website.
www.sbb.ch/content/dam /infrastruktur/trafima ge/bahnhofplaene/plan-zuerich-hb-a4.pdf
www.sbb.ch/content/dam /infrastruktur/trafima ge/bahnhofplaene/plan-zuerich-hb-a4.pdf Luggagecarts are free (but you need a 2 CHF coin or a Euro coin (forget thedenomination) to free the cart from the queue which is returned when youfinish and plug the cart back into another queue or rack. Luggagecarts go up and down escalators.
My own "short tour of Bern" leaves out a lot, but consists of a half day walking from the main station to the Rosegarden via the arcaded old city, including a digression out onto the bridge by the casino for a nice view, return to the old city past the bear pits and a meal at some nearby good places to eat and then return to the main station. The Rosegarden is not very pretty in December, but the views over the city are wonderful. A cafe, of course.
If you don't feel like walking back there are frquent buses from in front of the Brasserie Bärengraben
Very good menu, not cheap.
Another good place to eat, with easier acess to a table at crowded times is the old Tram Depotrestaurant across the street from the Brasserie.
Or there are buses with one change of bus from the back side of the Rosegarden. Tourist info in the main station (upper level) can help.
Montreux is a lovely city with a nice site on the lake and some beautiful nearby excursions. If it were earlier in the year I woud drop Montreux and spend the time hiking near the Jungfrau. However. many people would disagree, because Montreux is an excellent destination. So, that is just my liking for the Alps showing up. ;-) By December, hiking around the Jungfrau gets pretty cold, some trails and lifts are closed, and sunset is early. Montreux has a warmer setting on the lake and makes good sense.