Help with Alekhine's defence?

I've noticed in a lot of my Alekhine's games, there is a weakness on b7, and I always feel like I lose tempo to my opponent trying to cover it. I've created an example game (just made up) to illustrate what I'm talking about.

Obviously, I would never play this. I usually play something like 6:Nc6, but then after moves like b2 the white pawns seem quite strong, and then Bf3xNc6 is always an option to double black's pawns. I've thought about playing 6:a5, but I guess I'm not really sure what I'm supposed to be aiming for here. Could someone kindly help me to understand this, and how to proceed?

Is this playable?

I doubt it, looks like white can immediately play 0-0 followed by Rd1 on the next move and seems like a strong intitiave for the pawn to me.