>> What did you think of the rest of our itinerary? Ok or too much travel?
Well, you do have some long days of train travel, but also several stays of 3 nights to balance things out a bit. In the end, its a personal choice. If you have a limited time, some compromises usually have to be made, and what they are will vary from person to person.
Your plans for 19 Sep are quite ambitious. However, if you left Interlaken Ost at 7:29, you could be on top of the Gornergrat by 15:21 (with a bit over an hour to leave your luggage in your hotel or in the station lockers at Zermatt). The lockers at Zermatt are down a flight of stairs. There might be a lift, but I didn't see one.
A shorter scenic route would be via the Lötschberger route between Spiez and Brig: if you left Interlaken Ost at 8:30, you could be on top of the Gornergrat by 13:21, including almost an hour to stash your luggage.
To get that route, put in "From Interlaken Ost" "To Gornergrat" "via Kandersteg" "via Zermatt" and next to Zermatt, put in a stopover time of 30 minutes (to stash your luggage). Even though you put in 30 minutes stopover, it will turn out to be almost an hour due to the departure time for the Gornergrat train.
It will only be worthwhile going up to the Gornergrat if the weather is clear. If it turns out that the Matterhorn will be in the clouds that day, you might prefer to skip the Gornergrat and take the longer route from Interlaken to Zermatt via the Golden Pass route.
This is a good site to check the weather:
And here is a site with a webcam for the Gornergrat so you can see what the visibility is like:
You didn't mention that you would be trying to get VIP seats on the Golden Pass, but it might be worth mentioning in any case that they won't be available on your travel date.
From 3 Sep 2018 until an as-yet-undetermined date in 2019, carriages with VIP seats will be taken out of operation and renovated, so the train will just have the "ordinary" panoramic carriages in first and second class.