Ask the Druids from UK; I think they have a same problem. Nothing was ever written about druidry; all of their priests were killed
so they had to reinvent the religion from scrap. Is it all real or not,nobody knows, maybe those few who stumbled on some parts of the true druid path.
Since the Teutonic Order did such a good job on conversion,there could be a rather wide area of improvisation opened for a would-be Slavic religion priest; and nobody can say
what is right or wrong.