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.
I was unaware that you could input from a computer to a Garmin. Few, if any, stores have REAL sales people who are trained in the use and features of the products they sell. Garmin, at least when I sold them, was NOT famous for training sales people and their customer service, for the 'personal units, was about the worst in the industry. I have asked at several retail outlets about inputting routing from a laptop to a stand alone and the answer that I always get is, "DUH?".
I am not sure what you mean by extraneous information. I like my ETA, which is ALWAYS in either EDT or EST times. I like the mile countdowns to the next turn and final destination. I like the speed display. One bit of extraneous information I do like is the elevation. I just enjoy that.
I STILL want the ability to turn off the receiver. We use our laptop based system in that mode A LOT.
What I want is a 'hybrid system'. I want it laptop based with a smaller 'slave display'. Any changes inputted by my co-driver could then be seen on the small display. Both my co-driver and I would have access to the display, which I cannot do with a stand alone.
What it boils down to is that I cannot buy what I want, no one makes it.
There is much that I do not like about the laptop systems. They are not very good at finding places and often not up to date. For the most part though they fit our needs a little better than a stand alone.
I would have to show you how we do things in person for you to understand fully how we use our system. Maybe someday.