Creating customized routing on a Garmin (for example) is a little complicated, but it becomes easy and natural after you've done it a couple of times. Some units make it easier by having the "Routes/Multiple" feature. You simply insert waypoints, or stopoff points, to force it to calculate the routing you want. Which is the same thing you do on the laptop, actually. Many Garmins (like mine) only offer a single waypoint, so customizing routing is a little more tricky (but still pretty easy once you've learned the unit and how it works), but other units allow 50 or more waypoints. The GPS being ON or OFF is irrelevant when customizing a routing with waypoints.
There's also Garmin Mapsource, which you can use to input amazingly detailed routing on the laptop, and then simply transfer that data to the Garmin standalone. You can also do that with Google Maps by saving the route as a .gpx file and importing that into the unit. Both are more work than just creating waypoints on the unit.
There are lots of ways to do what you want, it's just that you actually need a standalone to do them, and you need a mid-to-high end unit, as the el cheapo's don't have the required features.
As for that big laptop screen, 90% of it is extraneous information that you don't need or even look at. Going from a laptop to a small standalone, the screen size was by biggest apprehension, which turned out to be not a factor at all.