There was an endemic fear since the Gaulish march on and sack of Rome in 390/ 387
It's something of an historical trope, but not one that's really that rooted down in Roman politics: campaigns against Gauls were largely driven by the same priorities and objectives that guided Roman against other neighbouring peoples as Samnites that is a general concern of military pressure as well as territorial control (especially on the fertile and rich Padan basin), and not a "preventive conquest". How Rome expands or not in central Italy is going to factor a lot IMO onto its expansion and politics in both southern and northern parts of the peninsula more so than an irrational fear of Gaulish peoples.
set up a Roman province around Greek city-state Massilia/ Marseilles?
No Carthaginian Empire essentially
means a different development of the Phocean cities since the Vth century BCE : now, there's no hard, definitive evidence of what Trogus Pompey considered the wars between Masslia and Carthage, but there's circumstantial clues that there were conflicts between Massalia (and local allies?) and local peoples maybe under Carthaginian employ (for instance the partial destructions of Lattara and the subsequent "switch" to Massaliote presence).
At this point, we could likely see the Hérault river as a limit between Massaliote and Ibero-Punic influence being butterflied away (possibly no Iberization of the peoples between the river and the Pyrénées (maybe even up to the Ebre?) and a broader Greek i.e. Massaliote, but Rhodanian as well since there's a fair chance the city of Rhodè might survives in situ without Punic challenge to control the "Gallic Isthmus"^.
While Massalia would probably have a better chance at establishing a presence in Corsica (no or different Battle of Alalia) this could potentially lead to a division of Greek influence between Marseille and its colonies on the Lower Rhône and coastal Provence roughly as IOTL plus maybe in eastern Languedoc and Corsica whereas Rhodè might have a comparable place in Languedoc and Catalonia. I don't think it'll necessarily end up with regular infighting, but it would make an interesting situation.
I wouldn't be overly surprised either if this lack of Punic and Ibero-Punic influence and a stronger Rhodanian presence wouldn't have a similar societal and structural influence over Gaulish peoples Massalia had, namely a drive for a network of commercial and military alliances directed at dealing with Greeks (a set of "Kebennoi" peoples along "Keltoi* in Greek geographies? Assuming both don't merge). A very interesting focus, nevertheless, I think no Carthaginian empire might have important consequences on the region , regardless if Roman history goes as IOTL or not,
(A rather maximalist take, I agree, and the depicted "trade influence" is definitely not something I'd see as mutually exclusive as Massaliote and Ibero-Punic presence on the Aude/Garonne axis is evidenced IOTL, but I'm lazy)
No existential threat of conquest for the Sicilian Greeks - do they unite , probably under pushy tyrants/ chief generals of the rising city of Syracuse, to conquer the Punic ports of W Sicily, or does the lack of an enemy to aid the rise of Syracuse (as leader of the Gk cities) and its generals push back the timeline - or end - the chances of Syracuse seizing the lead in Sicily and achieving a degree of Sicilian/ S Italian (forced) unity under Dionysius I?
What happens in Sicily is indeed pretty much an open question, but I wouldn't really see the Greek cities somehow united themselves against Punic establishment on the sole basis of origin and culture.
That said, what would you think of using Ancient Cyprus as a point of comparison? The island was divided in several city-states at least some had a marked mixed Greek and Phoenician aspect.
Maybe TTL Sicily could see something comparable (and, indeed, it's what partly happened IOTL under Carthaginian dominance) with Siculo-Punic cities being more broadly Hellenized and integrated within a Sicilian ensemble without being necessarily conquered while maintaining and claiming a strong distinct identity.
It's not impossible Syracuse would manage to get an even earlier upper hand on the island under Gelon's dynasty all the more without Carthaginian pressure (and maybe getting involved in the Second Persian War?), along with Agrigente or other important cities but even these fights would lead IMHO to local tyrants to search for powerful allies : if not IOTL Carthaginians, maybe ITTL non-imperial Carthage/Utica, Athens as IOTL, some other power, etc. so an Hermocrates-like figure remains likely.
That said, would Syracusae in a dual opposition between Carthage/Greek cities be as dominant it was? I'm not so sure, and either of the rise of a Dionysius-like figure whose own military power and focus came from this opposition.
Do the Punic trading cities of Africa bother with colonizing S Spain at all as they are disunited and short of a coherent leadership
I mean, Phoenician establisments in Spain already existed before the rise of imperial Carthage : Gades was reportedly founded ca. 1100 BCE for instance. So the problem wouldn't as much if they would still exist, but how deep and how far the settlement would go.
IOTL, Carthage mostly was content with occupying or informally "protecting" Ibero-Punic cities until the Barcid conquest, so I'd say the Alborran Sea would be pretty much a Punic lake altogether.
That said ITTL, I'd also see more successful Greek establishment on the peninsula as Mainake with a greater Greek influence on Iberian peoples along the coast (Ibero-Greek, I suppose?) while Punic influence over Spain's atlantic coast (and consequent societal/structural influence over Hispano-Celtic people) might be less important.
You made really interesting and stimulating points, thanks!