How to create level surface for cement board?

[HappyWarrior]

*Public health warning*: This is a very image-rich post, just to make it as comprehensive and context-rich as possible. May induce dizziness; please check with your insurance company before proceeding

Here is the fireplace as it originally looked (this morning!):



We are having an insert put in next week and the current fireplace isn't legal, so I ripped out the mantel so we can add a higher one, and re-tile with brick veneer and also build a brick hearth.

Here is how things looked after a morning's work:




I'm going to be using the original mantel top (higher, to reach the required clearance) and also add new, slimmer mantel legs.

I also need to repair the big hole above the old tile, where the drywall was damp and ripped away.

Time for another picture! New additions will look like this:



Next planned step is to add cement board and tile the below area with the new brick veneer:




*It's at this point in the post that my wife looks over my shoulder and reminds me that it's "mantel" and not "mantle". For clarity, I'm not referring to the Earth's crust....

The issue I have, as the below photos show, is that the brick sticks out quite a ways from the old drywall (and the new drywall piece, when installed). It was well hidden by the old mantel legs, but won't be after I put the new slimmer legs in:





Finally...heck...my question!

When the new matel/legs/drywall are installed, how can I level out the rest of the surface so I can fit the cement board to the cross-hatched area, a few pics above?

Is there some kind of mud I can apply to cover both the drywall and the brick, to smooth into a single surface?

Or should be doing the entire thing another way? I've never done anything else like this before in my life, so it's all new.

Thanks!

 

[Rearscreen]

I think you mean "plumb" to the surface. My first thought would be to have a larger surround bump out to match the farthest out brick. Make it decorative. As if it was meant to be. Otherwise you'd be grinding down the bricks and your wife would be grinding you down from the dust created.

 

[HappyWarrior]

Good idea. Would it be even easier to just keep things as they are and build a steel stud frame to add the cement board to, like this one?

 

[Rearscreen]

There you go! Bam!