I too love long travelling days by train in Switzerland, soaking up as much as I can with an Interrail ticket - but hat's off to you - that's an ambitious day's tour!
However, if you want to cut it back a bit, but still enjoy many of the main elements, here's another idea to consider. It cuts off 2.5 hours from your original plan, leaving Zürich HB after 8am and getting back before 8pm. It omits the eastern and western legs of the Chur - Zermatt route, but includes all the essential bits in the middle, and even includes some new routes with spectacular scenery. Also, you can adapt it once you get beyond the Andermatt stage.
The main outline plan involves a loop Zürich - Andermatt - Operalppass - Andermatt - Brig - Kandersteg - Spiez - Interlaken Ost - Luzern - Zürich, and takes in some of my favourite, most spectacular, corkscrew and scenic routes in Switzerland. Even if the weather is unkind, there's plenty to excite the senses, and plenty of breaks!
I've cobbled together this adaptable timetable, but it's always wise to check! It should just about be available on any day of the week...
Zürich HB dep 08.05; Göschenen arr 09.50
(direct train, this will probably be one of the posh new copper-coloured Treno Gottardo trains)
Göschenen dep 09.54 (or 10.12); Andermatt arr 10.06 (or 10.22)
(notice the original main line Gotthard tunnel portals at the south end of Göschenen station, which the Treno Gottardo will plunge into on its way into Italian-speaking Ticino canton. Your train climbs 1,000 feet from here on its short journey - plenty of engineering to marvel at, from the LH side of the little train)
Andermatt dep 10.28; Oberalppass arr 10.47
(a further 1,000 climb to Nätschen, via a bewildering set of switchback very tight curves to gain height - you can actually view the tortuous zig-zag route up the hillside from Andermatt station). You could delay here further, or as 1960man suggests, walk from Nätschen to Oberalppass or v/v. The beauty is that you can decide at the moment. If you miss out stopping off at Nätschen, you can probably get a decent view anyway from the train. I'll continue with my original scheme...)
Oberalppass dep 10.53; Andermatt arr 11.22
Andermatt dep 11.37; Brig arr 13.33
(Excellent place for a lunch break and a bit of sightseeing. If you want to delay further anywhere up to this point you can dash back to Zürich from Brig in 2.5 hours, via a long tunnel and Bern, but it would miss so much of Switzerland's most impressive scenery from the train. May I strongly recommend the following?...)
Brig dep 14.36; Spiez arr 15.44
(via the original Lötschberg mountain main line, which climbs spectacularly out of Brig - terrific vista over the Rhône valley, then encroaching jagged mountains, the summit tunnel and the lovely Kandersteg valley, followed by the descent to Spiez via a series of bewildering reverse curves, tunnels and confusing directions
Spiez dep 16.05; Interlaken Ost arr 16.26
(the railway skirts the south side of beautiful Lake Thun on this short run)
Interlaken Ost dep 17.04; Luzern arr 18.55
(one of the famous scenic routes, the eastern leg of the so-called 'Golden Pass' route, via Lake Brienz, Meiringen and Sarnen)
then a quick dash back to your starting point...
Luzern dep 19.09; Zürich HB arr 19.50
Almost all services are at least hourly.
I hope there's something there that may appeal, though of course you may have seen some of these locations before. Many of these places are on my list of spots to revisit when I take my next 'Global' Interrail trip in September - despite its name, and its availability stretching from Finland to Portugal - I shall be using it mostly in Switzerland.
Good luck, and enjoy the ride
1) engineering feats between Göschenen and Andermatt
2) at Andermatt station you can trace the 'Glacier Express' route leaving to the right and zigzagging wildly up the hill; the Göschenen train is coming in from the left
3) spectacular view west into the Rhône valley, from the climb out of Brig towards Kandersteg
4) lovely Kandersteg village