1) court documents in NY show that he stood trial for being a "disorderly person" for treasure digging
2) Isaac Hale (Emma's father) (IH) recorded about JS treasure digging and it's in Joseph's own biography, that he came to live with the Hales while employed in the area in a treasure seeking adventure. That is how Joseph met Emma in 1827, and Isaac did not want his daughter marrying a man with that unreputable trade. Aside for IH's account you can read it in Joseph's own account, and in that of neighbor Willard Chase. The local congregation that Joseph and Emma attended (yet he was told to join no church :), the First Vision story changed over time in the 1830s) had members who protested his attendance, as his profession was that of a treasure digger. IH offered JS a piece of his land and the house for Emma and Joseph to live in (where JS completed the "translation" w/OC") if he would give up his treasure digging and become a respectable farmer -- but religion became Joseph's mode of pay (vs the menciant lifestyle that Jesus asked of his apostles when he sent them forth)
3) affidavits of Palmyra citizens note that JS Sr.'s profession was "treasure digger" and "farmer"
4) D and C section about Joseph receiving a revelation to go to Salem, MA and look for treasure
Understanding that the Smiths were treasure diggers and believers in folk magic, fits the BOM story well (gold plates, a cadence to follow before getting the plates, a seer stone, an angel protecting the treasure, coming back each year), etc. Also, understanding that 1820s were a time of mysticism and folk magic in general (Sleepy Hollow was written in this time, etc.). We look down upon "treasure digging now," and it wasn't a respected profession then -- but it wasn't as big of a deal then. I'm not excusing it, I'm just saying that to understand the time, context, place -- helps to understand the story of the Smiths
I think Joseph was involved in treasure digging, he was looking to get money from little to no work. He was looking into his stones and telling people where to find treasure, though they never found any - until he found his own. I think the hill cumorah story of Joseph obtaining the gold plates has changed over time and originally fits very close to treasure digging myths of the day. Over the decades the official church narrative has cleaned up and sanitized it - for example, in early recordings, he found the plates by looking into his seer stone (which he then used to translate the plates). The foundation stories of the church fit exactly into the folk-magic of his day.