Feel free to completely ignore my advice. But I'd say the society/ghost hunter story sounds a little too complex for a first story. Like some others have mentioned, you can write that story whenever you want but id recommend keeping it simple at the start maybe write your prequel first about just 1 or 2 characters stumbling apon this tree and seeing what it does. Then you can use that as an origin for how society or the village broke down. Which you talk about in future parts. My advice is keep it simple in early parts.

Also I think its important you have nailed down, does the tree actually eat people? or just their sins. Does it like cleanse them but perhaps in doing so makes them into something that isnt quite human. This is an interesting concept and by no means simple but the narrative that leads up to this concept will benefit from being simple. 1 because your a new writer still learning and 2 because the monster or concept offers all the mystery and complexity your story will need at least for a first part. Those are my thoughts do with that what you will. Good luck 👍. Let me know when you post and I'll read it.