HAi,
I am using a TomTom rider 400, and before that a TomTom Rider II, and before that a Garmin etrex.
I plan all my traveling quite thoroughly at home with MRA (and before that Tyre), on a pc with two 23 inch screens (bit of a computer freak you see). That includes places to visit, hotels or bread and breakfast ... During a holiday I usually don't want to ponder over route changes, or alternatives and ... hence I almost never have the need to change a route. The only thing that regularly happens is that we spend too much time on one or another visit and then skip the last part of the track for a direct route to the hotel.
I completely follow your thoughts about smartphones on a bike. In a car; yes, on a motorcycle; I fear that those tender things will break quite fast but there are other people here who seem very happy using their phone.
It would become completely different if I could replace the TomTom with a rugged tablet. Bigger screen, android, and all tablet functions next to a GPS would avoid having to take an extra tablet travelling that is now needed to keep contact with the home front. Someone on this site has put such a thing on his BMW and seems very happy with it.
For your case, you already have a Zumo, and you can prepare your routes with MRA. I think that this is more or less a perfect combination. I would keep the iphone in the pocket and only use it when the Zumo would unexpectedly die on me.
I hope this helps