Berlin to Füssen can be done with just one change. You don't need a reservation. Most of the time there are two trains per hour if you are willing to do a second change. That's no easy way? (Füssen to Zürich can also be done without reservation with just one change.)

Nightjet is included, but needs a mandatory reservation. 

Thank you for your reply. I suppose one change isn’t too bad, I will consider :) 
Is there ever any risk relying on a train that doesn’t require reservations (i.e. if the train is already full is it possible you wouldn't be allowed to ride on that specific train)? 

The main risk is that you'd get no seat and need to stand. But it's an open system. And people do get on and off the train all of the time so seats do become available. 

And changing trains in even a few minutes is normal. It's just hop off, hop on. 

That the long distance day trains in Germany or Switzerland don't require a reservation doesn"t mean that you can't get one. It's possible to get from 3 €. 

BTW: If you consider to go by Nightjet: Berlin to Zürich is well booked most of the time long ago (especially if you'd like to get a sleeper).