Johnson will make equally dangerous policies, but they'll be well thought out?
They may well be dangerous, but they won't be thought out. Well, not be Johnson anyway. (By Cummings, yes.)
I agree with RhinoskinhaveI that Johnson isn't in the Trump league of ineptness, but he is inept and does have the same lust for a crown and contempt for the rule of law that Trump has.
That's the bit that makes him dangerous, because Johnson will do whatever he believes serves himself best, and not care who or what gets broken.
One of the differences between them, though, is that Johnson understands there may be some consequences if he's caught; Trump's tiny brain can't really grasp that, other than as a reptilian, in-the-trap-right-now way.
Another is that Johnson understands (brags) that he's lazy and incompetent, and drafts other people in to do the real work. He does have a vague understanding that there is some work to be done –although he underestimates it because he's never done it.
Trump doesn't understand that he's incompetent, or that there is any work to be done. He thinks he bangs his magic fist on the desk and shouts, and the world will change around him.