You only need to look at what FB has said and done over his entire career in public and private, his copycatting of Trump (ahem, the first debate), and the conduct of him and his father's government (stealing funds allocated for social programs for a shadow budget) to know that his government, with executive, legislative, and judicial majorities, would wreak havoc on every Brazilian instution possible, ala Trump's second government.
As is currently observable in the United States, given consolidation of power in the three branches of government, it is no longer necessary to follow the law or the constitution. Minimal research into who from the US government the Bolsonaros associate with tells you what their agenda would have been in the "Bolsonaro 2" government: complete, transparent self-interest and complete disregard for actually administering the country for the benefit of the people.
Taking him at his word in the last three months of the campaign after 20 years of a vapid/nothing political career full of police investigations and associations with organized crime makes no sense.