Game Tips (Elden Ring)

Lesser Tree Erd: Was this boss needed for a quest? Killed him, but I don't know why




I'm playing as an astrologer. Does anyone know where to learn/buy new spells? I found only one NPC from the school of magic who sold the spells that I already had. Throw a screenshot where you can find the right NPCs.
It would also be nice to have a new staff and armor more abruptly than the starting one. Also screenshots pliz if you know where to get it. And by the way, does it make sense to improve the staff at the blacksmith?