(even Cersei confirmed that giving Stannis Dragonstone while Renly got Storm's End was intended by Robert as an insult).
Not exactly. I recall the problem was that Renly was still basically a child when the war ended, and with Dragonstone's previous lords obviously vacant, Robert needed someone
to manage that estate - someone he could trust. Stannis was needed
there - the right man for the right job - but a consequence of giving him that title was letting Renly have the more lucrative and prestigious Storm's End. It could be Robert's attitude towards Stannis insisting on Storm's End got more sour later, but the initial posting was not an insult.