My view is.. if you want maps use a phone!
There are cheap ones out there so no need to spend the earth or risk a posh one on the bars.
I intend to fit purion and either mount my garmin centrally or buy a throw away phone and use that.
Not sure how seeing the assistance level helps....
why would I use a phone when I have a GPS?

it helps understanding how the assist works, you get a feel for how hard the motor push, really interesting to see, on my fatbike not much assist is needed going straight on asphalt, but when I bike just beside the road the assist level doubles. That is nice information to have I think, especially if you're going on long rides. But not necessary by any means of course
Actually, if you're having a Garmin beside the Garmin display is not necessary at all. You have the power meter on the battery pack. You have +/- buttons. The display could be removed completely. It would be cool if there was such a version and the Bosch system sending the data through ANT+ to your Garmin instead
