Turn edits are a good way to stick to your own way,
also putting in some Waypoints helps-all you need is a fast food place near your preferred exit or hiway.
Also try this, maybe this is the most you can get out of the Nav route setup.
Go into Nav settings
Go to Route Preferences
is "Always Use Preferred Route" selected?
This is why it wants to stay on a route and route back to it if you leave the route since you know your own way.
Deselect that option.
Now when you pick a route and set as Dest, it displays in a small box on the the route screen all 3 options as buttons;
Fastest
Shortest
Eco route
Each button has a different colored dot that will be used for road lines if you pick it.
You can toggle among them and it will display the chosen route category highlighted on the map
Pick one and it will use that.
When I tested it :
My particular route was the same for all 3 options-it was only 3 miles away so not a long enough route to really see what it would pick.
When I left the route, it seemed not to keep trying to route back or turn back towards the route behind me, but led to the chosen route as I kept going, back to the route but along my current way.
Seems better so far.
...yes I needed the Nav to go 3 miles...they will put a store anywhere nowadays...