A more complete conquest of Iberia wouldn't neutralize the strategic surprise of a march over the Alps, while providing Carthage greater strategic depth, something it badly needed OTL. The Romans had trouble dispatching more than ~20,000 men to overseas theatres, so if the Carthaginians can keep a 30-40,000 man army cantoned along the Ebro, the Romans may well find it impossible to make headway against Hannibal's strategic base.
The drawback of waiting isn't a loss of surprise, it's that while Hannibal is conquering Spain more thoroughly, the Romans will be conquering Cisalpine Gaul more thoroughly, making it more difficult for Hannibal to recruit enthusiastic allies when he completes his march. This need not be fatal, though. He may still succeed in drawing sufficient allies at a later date, or maybe there's less attrition during his march to and over the Alps, so he arrives strong enough to beat the nearest Roman army and carve out his winter quarters without them.
Also, there's very little real difference in centralization between Rome and Carthage; they're both alliance systems based on a web of bilateral relations of self interest. Indeed, it's possible Carthage was more centralized at this time, having established direct administration of a considerable area in Africa. The real difference is that Carthage drew a greater share of its manpower from the periphery, rather than the core of the empire, which made territorial losses there more dangerous, but this isn't an insurmountable issue.