Their primary concern for the presidency was not destroying the union. To them, a "rogue" president would be someone conspiring with the British (etc.) to overthrow the government. And the punishment for treason is very clear. They didn't need to add anything.

Bear in mind, the founders' intention was a much smaller federal government than we have today. When the federal government's powers are limited, the president's power is also limited. Many of the political issues that we argue about today were not intended to be federal concerns -- and, in fact, were not federal concerns for a long while. So you suggest that the founders might require income tax disclosure, but my guess is they would have viewed the imposition of any income tax as tyranny (absent dire circumstances).