I would agree about similarities between them all. The Scandinavian religions didn’t leave written material either, but military records and christian sources do give us some information about them. Such things likely exist about the Celts.Druids formed the priest caste in indigenous Celtic society. They likely had very specific teachings and religious customs but they were wiped out quite hard during Christianization. More than any other native religious tradition I'm aware of
We can conjecture a lot about them though from the puzzle pieces that remain. All Proto-Indo-European religion is quite similar beneath surface level labels