Looking at your itinerary, the morning Brussels to Frankfurt ICE train for 2 days from now is showing high demand, so I would definitely get a reservation for that.
For the Frankfurt to Rothenburg, the ICE trains to Wuerzburg are showing medium to low demand.
For Rothenburg to Munich, depending on how you go, most IC and ICE trains are showing low to medium demand.
All the train from Zurich to Interlaken are showing medium demand.
Spiez to Visp trains are showing low demand (the only leg to Zermatt where reservations are sold)
Zermatt to Geneva is showing low demand. None of the trains to Geneva from Visp sell reservations.
So I really don't think you need reservations except for the Brussels to Frankfurt ICE. Other trains, you can check the expected loading a few days ahead and buy reservations if you like. In Germany, you can buy them at train stations for 4.50 EUR per seat. Swiss trains are so frequent that you almost never need reservations.