In short, we can show that Joseph and/or his compatriots were involved in intentional deception, explicit plagiarism, and attempts to bury evidence of misdeeds. I don't think we can ever completely rule out psychosis or an epic level of self-dillusion, but I think it highly unlikely considering what we know about his actions and behaviors. There's too much to write to go through it all, but I'll touch on some of the evidence below.

1. As you mentioned, Joseph had a long history of cons and frauds. This started young as a failed seer or glass looker. He was involved in at least one expedition for buried treasure, along with his father. This produced no results. Every time they started to dig, Joseph would say evil spirits took the gold away. He was involved in a fraudulent banking scheme, which he fled from. He was involved in several secret and illegal marriages across multiple states, which he publicly and repeatedly lied about. There are others, but I mention these because they required an active attempt at deception. See more information here, and a copy of his youth arrest for scrying here.

2. Joseph's origin claims are demonstrably fraudulent. For example, the Book of Mormon claims to have been completed in 600 BC and translated solely from that record in 1829; however, several verses were copied verbatim from the 1611 KJV + Apocrypha (his family bible), including translation errors of the KJV translators (Joseph's family bible). There are many other examples of plagiarism, but this one is the most blatant. Again, something that required an intent to deceive.

3. Joseph had a history of hiding evidence of his misdeeds. For example, we have one letter he wrote where he told a potential lover to burn the letter to protect him, one of his former wives accused him of ordering abortions after impregnating a girl, and ordering his secretary to burn the minutes of his infamous council of 50 (his attempt at the presidency) when arrested for treason.