I have a garmin in a soft case, and the same issue as you. I did find that a 3 year old iPhone battery is much more cold sensitive than a new one- you may be able to use your phone if you replace the battery. For my lake, the Navionics maps are better then whatever the Garmin has on it.
I do use the Garmin in the soft case for navigation though. The case has 2 plastic rings on either side, and I found command hooks that had a swinging metal hook. I placed one on each side of the dash, and hook the hooks on both sides, and the soft bag rides in the dash where I can see it. I suppose it depends on your snowmobile, but I assume you have space if you are looking for a mounting solution.
Can't see from the pictures, but the hooks swing on a pivot, and I swing them over the rings on the case, and they keep the case in place as fast as I am going.