Is this a good opening?

The black bishop retreating to either a5 or c5 is worse for black than it first seems, all white has to do afterwards is push to b4, forcing the black bishop to further retreat all the way back to d6, blocking the d pawn and thus stopping the light squared bishop from developing. On the other hand, if after b4, if black retreats to any other square, black will lose a pawn, as white then further pushes to d5, black will have to retreat the knight, causing the pawn on e5 to be taken for free.

I think Black has pretty visible compensation here, if this is what you're talking about.

Is pushing d4 better for white than pushing b4?

Yes, if that wasn't clear enough

What is whites main goal after pushing d4?

Please watch a video on opening principles. That'll answer a lot of your questions