If you don't do routes and navigation then you'll probably have a learning curve with some gotcha's that will crop up when you do start in the lower 48. However I have but don't use the navigation feature or routes on my Garmin's either. And I live where there are plenty of roads to chose from. If you are the type that can look at a map before you ride and know what you need to do, then you'll be all the better just doing that.
But if you need routes and navigation, just remember you'll have a few unexpected things happen. So practice while you are where you know the way.
Garmin has methods for dealing with both watches and Edge devices tracking the same ride. If you use Garmin's phone app, it'll probably pop something up about that when you pair your Epix to it. The watch will be useful for getting information about you while off the bike if you wear it all the time. There are some here that routinely use Garmin watches and Edges, I probably am getting something slightly wrong since I don't use a watch. So look for their replies.