Hi. I hope you’re enjoying the solstice period.
I think there could be some problems with your hypothesis. The Romans didn’t really get on with the Gauls or the Celtic peoples at large for most of their history and they certainly didn’t like druids. A Roman knight was even put to death for having a “druid” amulet during a court case and druidry was banned by imperial decree. You also notice that the Romans, as a whole, tend to be far more hostile to the druids/Celts/Gauls in their writings than the Greeks. Another problem is that Celtic-speaking peoples populated vast swathes of Northern Italy from early times, but there is no mention of druids among them by any classical writer, likewise, there seems to be no mention of druids in Celtic Iberia either.
The Germanic Anglo-Saxons, had their own pagan beliefs that may have borne some similarity to pagan Celtic beliefs, but there is little to suggest that they had druids, the word drycræft exists in Old English, but it seems to have been adapted from either Old Irish or Old Brythonic. The Anglo-Saxons in England, weren’t pagans for particularly long before they converted to Christianity and by the time the Viking raiders started to arrive, they were thoroughly Christian in their outlook – albeit with many older folk elements mixed in. Furthermore, they didn’t really get on with the Welsh, Cumbrians or men of the Old North and the West Welsh or Cornish – who by that time were part of Romano-Christian culture. The Norse cultures had seiðr, but I don’t think that we can call them druids and the attitudes towards the seiðr were very different from the attitudes towards druids in Celtic societies.