I think you can use the term druidism. It seems to be a ubiquitous term in the fantasy genre already. Also, incorporating practical everyday knowledge like herb lore in role play isn't really "stealing" from a culture, it is just a fact. I'm pretty sure you aren't stepping on cultural toes by using lore that is practical. Like you said, it's the mystical regionally specific things I feel you need to wary of. For example, Celtic druids specifically worship the goddess Brigid. She is one of their most revered goddess and is in charge of healing, poetry and smithcraft among other things. In deference to the actually religion, make up your own strong female goddess and her attributes.
As for astrology, I'd use the base knowledge - Sagittarians are free spirits, Libra seeks balance, etc. - and just give the constellations, stars, planets new names. I think that western astrology is also part of the public domain, and you won't be antagonizing anyone who wasn't already looking for a fight.