I think it is useful in these sorts of problems to categorize moves. You say that Re5 looks promising, but that there are too many moves for black afterwards. I would work through this as follows:
What's good about Re5? It prevents the opponent's king from moving, shrinking the possibilities we need to consider. The threat is 2. Rc5#, but this is prevented by black's knight. If knight moves, 2.Rc5# will be possible because there are no checks. If pawn moves Rc5# remains impossible and no other mate presents itself.
Note that I don't need to consider where the knight is moving to. Since we know our threat after Re5 all we have to consider are moves that prevent that threat from working. d2 is such a move, and tells us that Re5 doesn't work.
To find the correct solution, we need to find a different solution to 1... kb5 or kd5. Blocking with the rook of queen like you've proposed doesn't work, so we have to instead look for situations where those moves are mate in 1. This leads us to realize our bishop being in the way is a serious problem, which motivates 1. Bb6, which is indeed correct.