I don't see much cargo plane with a long range so I wonder if a stopover affects cargo like when it affects passenger?
1°) Tech-stops don't affect cargo. A direct flight might still have an advantage over yours, but at least you'll still have 100% LF if unopposed.
2°) Freighters' nominal range at MTOW is always lame, but you got to figure this works in a different manners from pax ops. You usually can fly until the distance you got at half MTOW and still be completely profitable. Depends on the operational costs of each bird.
For example, don't tech-stop a 1700 nm route with a 732F only because MTOW allows a max range of only 1200 nm or so. Fly direct. You'll lose a little capacity, but that's all.