Do you want to create your own routes while you’re out? FOr multi day touring type stuff? Or changing your plans from the ones your started out with?
For planning routes in advance, you’re much better off doing on a laptop before you leave. bikehike is the simplest, you just carry on adding points on the map to draw your route, and you can switch between Open Source cycling maps, satellite photo and Ordnance Survey maps.
Garmin basecamp is more flexible and easier if you’re familiar with it, but I don’t use it often enough and have to relearn when I do use it.
When you’ve finished the route, save it as gpx file and it goes on the device just like anyone else’s route and then you can follow TBT or line on the screen, however you prefer.
For mapping on the device, I use openmtbmaps. Contours, zoomable, turn-by-turnable, clear to view.