Which add on? Lugano, Appenzell, Montreau, or Bern

I recommend that you cut Chur/St Mortiz. They are located in the east side of the country and requires some time to get to from Zermatt. I suggest you spend some time in the French speaking or Italian speaking part of the country. From Brig, you can go to Locarno in Ticino or you can go to Lausanne/Montreux. St Moritz in winter is like Vail--a place to see and to be seen. However, if you want to embrace nature/the Swiss culture, there are better places.

I fully agree with leaving out St Moritz/Chur. After Zermatt you can head straight to the Montreux area for maybe 2 nights, then head to Bern for 1 night. My favorite route from Montreux to Bern is the ferry to Lausanne followed by the train. You would miss the Golden Pass, but can always ride it as a half-day trip from Montreux if so inclined - personally I don't think it is that big of a loss, the part from Luzern to Interlaken is stunning already.
If my count is correct you would still have a spare night or two, which could be spent in the Grindelwald area, or in Luzern.

@susandealy. It's a bit challenging to provide recommendations without knowing how you like to spend you time (i.e., city exploring, walking/hiking in countryside, museum hopping, etc.). My quick two-cents here: Appenzell is a superb area for hiking and not to be missed for those who enjoy the outdoors; Bern was a wonderous big city visit; Lake Como was unforgetable vs. Lugano; BTW, two nights in Zurich (at the beginning of our trip) and it's old town and lake were a fantastic beginning to our three week visit. Safe travels.

You could stick to the 4 areas you've already picked and add the extra nights in those places for a more through experience. I've learned you can't see it all and it's a mistake to spread yourself too thin.

Well it seems to me the Glacier Express trip is the critical part since it takes you at least to Chur from Zermatt. So if thats a must, I'd go from there up to Appenzell area. Maybe a critical question is where (if anywhere) are you going after? If flying home, then circling back closer to Zurich (i.e., Appenzell) would be ideal. If going on to Italy or somewhere south, St Moritz area (the Engadine) or the Lugano route. But perhaps a bigger factor is all those one-two night stays, which leaves hardly time to see anything, so maybe it doesnt matter. If the trip is mostly about the train rides, then that makes a difference.