Something that occurred to me is this; I often hear about how the Romans had a policy of incorporating other gods into their pantheons by saying they were just other forms of their own gods. People frame it as being totally peaceful and a great thing...
My problem with that is, wouldn’t that be cultural appropriation? Wouldn’t it have caused changes the religions of the natives, trying to force them to conform? I want to point out we didn’t see any of this with the Mongols, or Alexander the Great. Both just let the people worship as they pleased, to my knowledge. Yet Rome’s policy of integrating other deities into their religion would entail forcing the worshippers to worship their gods.
So has Rome’s treatment of the Celtic religions been mischaracterized and wasn’t benevolent? Could they have been a reason for why there’s little first hand sources for it?
This reply comes rather late, but I hope you might learn something from these earlier threads.
To begin with, the Romans generally did not force conquered peoples to adopt Roman culture or religion. Instead provincials tended to romanise to get access to Roman economic and political networks (as well as influence from nearby military settlements) as u/Alkibiades415 has explained here. An interesting example is the imperial cult; as u/PippinIRL discusses in a reply to a great answer by u/MagratMakeTheTea, often the emperor would approve worship of themselves when local elites already had of their own accord built a temple. Of course this was an unequal cultural relationship, but it was not based on state force as much as one might think. Other thread I can recommend on how Roman syncretism worked is this by u/tinyblondeduckling and this by u/cleopatra_philopater, specifically on the Isis cult
When it comes to the Celtic religion specifically the Romans did to some extent persecute druids, which might have been due to their practice of human sacrifice or their political prominence; u/Libertat (here) and u/Steelcan909 (here) give two different perspectives on the issue.