IMO (and experience), if you want to specify the details of a route, it's best done by laying out the route on your computer on sites like RideWithGPS, GPSies, MapMyRide, etc., then use their apps on a smart phone to navigate it, or export it in a suitable file format for a Garmin Edge bike GPS. Some GPS devices themselves have the capability to automatically create routes between two points but offer virtually no control of the route selection except for choosing some general exclusions like avoid toll roads, highways, etc.. I've never found this to be satisfactory.

In the past, I have laid out routes on the RWGPS website using a smart phone and then downloaded the routes to a Garmin. About a year ago RWGPS "updated" their site and it no longer works correctly on any phone browser I've tried. Other sites may work, but I haven't tried them.