looks really nice! the only thing i can say with any certainty however, is negative... the celts having an ethnogenesis in central europe is an outdated model. its just the art that came from there, we dont know where exactly the language and religion came from, but it's now thought to be from elsewhere, maybe even the atlantic bronze age culture but thats just a guess based on how there's no evidence for an invasion into britain/ireland from this time. really its a nitpick, because those guys are still going to end up celts, and are still going to pass on their material culture to the celtosphere. im all for adding in archaeological material cultures to the otl maps, its just that i think we should be careful about the colouring
Somewhere in France probably, like you mentioned possibly the Beaker Culture. I mentioned northern Italy and Spain because there the most ancient proof of a Written Celtic language have been found, namely in Spain some tombstones (dubious attribution) and a drinking cup near Arona, Northern Italy (the cup itself, or a reproduction, is exposed in the Archaeological Museum in Arona; I mention that because I live in the same zones, I've actually seen the cup so I'm talking about the things I know best). Since, the Celtic origin of the Tombstones is not certain, the Arona cup is the most ancient example of written Celtic that we currently have.
Basing myself on what I've seen and read in the Museum's exposition, the Insubrians where a Celtic people coming from Eastern France, that merged with the pre-existing Golasecca Culture, the more advanced local culture of the Bronze Age (the Castelletto Sopra Ticino Museum shows something about that too, though it lacks such a clear exposition as that of Arona).
The Insubrian Kingdom is mentioned as such by a latin Historian, Livy if I remember correctly, as founded by a "King Bellovesus" sometimes around the time of Servius Tullius. It extended north of the Po river, between the Sesia and Adda River, all the way to St.Gottard Pass in the Swiss Alps. The Golasecca Culture is 400 to 600 years older, and extended a little further East, to the River Brembo for sure and maybe, just maybe, the Adige but don't quote me on the last tidbit.
There was an even more ancient Canegrate Culture, but not much more is known about that.
EDIT: Bellovesus is mentioned as the founder of Milan (Legend, unconfirmed). Said foundation was dated around the 570s by an article in an article about archaeology that appeared in a local paper, though that's mostly speculation on the writer's part and is not mentioned anywhere else