As explained on the opening page of this following link, the SBB computers default to the fastest route. To force it to slower routes ( which is usually the correct descriptive word for the more scenic routes), you have to put in a "via" station:
That is why I listed the stops along my other suggested routing.
To clarify what I 'm about to discuss, you would do well to have a map open.
The very scenic route from Montreux via Zweisimmen to Spiez ( along the Golden Pass tracks) at 0844 arrives Spiez at 1147. 3 hours and 3 minutes. Continuing to Zermatt at 1212 via the very scenic upper route through Kandersteg gets you to Zermatt at 1451. That does not meet your scheduling needs.
Just for amusement I let the computer get me to Spiez from Montreux by the fastest route. It can take 2 hrs and 14 minutes via Lausanne and Bern. If you go to Visp via Martigny, using the Rhone Valley part of the way that you plan to, then take the fast train through the new fast basis tunnel to Spiez, it actually takes only 1 hr 46 minutes. Just a bit over half as much time as using the Golden Pass routing. That points out how much time is saved by using mainline trains. I'm not suggesting that route for you.
If you were to compromise, and go from Montreux at 0844 to Spiez via Zweisimmen ( slow route, Golden Pass tracks) then use the fast boring route through the new "basis" tunnel to Visp and onward to Zermatt, it would take an additional 1 hr 47 minutes to get to Zermatt. You'd arrive at 1351. Possibly, too late for you; maybe not?. You'd have to leave Montreux at 0844, a couple of hours before you plan to.
The 1038 departure you suggest from Montreux gets you to Zermatt at 1314.....2 hrs, 36 minutes. Obviously, you could go earlier.
This is an exercise in trading off scenic travel on slow trains, vs. fast travel on less scenic trains and having more time when you are at your destination ( or, for a late breakfast).
Swizerland rail gives you those choices.