ر is made near the tongue tip against the upper gum area. It is close to nun's point but differs in tongue movement and manner of sound.
In رَبِّ, pronounce one clear, controlled ra. Exaggerated repeated trilling that produces several audible ra sounds is not the goal; nor should it collapse into lam or nun.
The weight of ra concerns its letter qualities; the present focus is identifying its correct articulation point.