If you want something non-drawish play the parham and win in 20 moves.
You can't play Parham as black though.
To the OP,
Ruy Lopez exchange is winning for white, so I don't see how it's "drawish". Black has a crippled Q-side pawn structure, while white maintains a healthy 4-3 majority on the K-side. White will gradually trade off all the pieces, create a passed pawn on the K-side and win in the endgame. Black cannot force the creation of a passed pawn on the Q-side because his c-pawns are doubled.
You make it sound so easy, i have found it very difficult personally to contain black's bishops. If the spanish exchange was winning I doubt we would ever see nc6 played as black again.
I'm not saying it's easy. But in theory, if white makes no mistakes (of course we are all human so we make mistakes) the position should be winning for white. But as you said, there are so many ways to mess it up.
Also, if RL exchange is winning, why wouldn't black ever play Nc6? There are plenty of other responses besides a6 after white plays Bb5.
The argument of in theory never works, you say that if white were to make no mistakes than in theory it should be winning? How can you even make that argument how do you know that blacks bishops in a opening position don't give him the forced win?
If you're looking for a dynamic response to 5.0-0, 5...Ne7 leads to double-edged positions that you will probably understand better than White (if you've studied the line!). It is discussed (as a backup line) in the excellent Ruy Lopez: A Guide For Black by Johnsen and Johannesen.