Ensure smooth drywall surface around stone mantel

Question

I have a follow-up question from this Frame drywall around brick fireplace regarding the "best method" to ensure smooth drywall surface on top and both sides of mantel.

Basically I have a very "rough" brick fireplace and I need to put drywall in the "green" area:

The "red" area is the mantel and the "yellow" area will be tiles.

The drywall will attach to horizontal metal studs as below which shows the "bottom" studs and no more studs under that one:

My question is: I can mount drywall on the studs and ensure they are flat on the top portion but how do I make the drywall flat for area on both sides of the mantel (label as #1 and #2)?

The bricks are very uneven and the whole fireplace is leaning "outside" a bit so it's not completely vertical.

I am thinking of 2 options:

1. Put mortar in #1 and #2 and try to ensure it's 1/2" below the top drywall. I am thinking of running several small strings horizontally to ensure they are flushed and parallel to the drywall. Then once that's dried, I can put drywall over mortar using concrete screws. However the hard part is to make it perfectly 1/2" depth. I could "mud" heavily the entire thing later as a finishing work.

2. Frame #1 and #2 but using lumber and put several "shims" and screw plywood in first, then drywall covering plywood. I don't know how exactly this will turn out but it will take several attempts to make it look right.

I am looking advise. Thanks.

UPDATE 1

Following advise below, I have two options to mount the metal studs:

1. Do one horizontal on bottom. Maybe the drywall is strong enough to go from top to bottom of the two studs?

2. Do 2 vertical studs like the photo for the left side of the fireplace as example:

Right:

Left:

(I haven't cut the metals yet hence two separate pics)

The second option seems harder to "line things up" due to the bricks BUT I can bend the metal and use screws/washers to position.

Let me know.

UPDATE 2

To @Jack credit below, I was able to complete my project and it looks beautiful

Answer 1

Cut and set 2 vertical studs screwed to the horizontal stud by bending tabs to attach to the underside of the horizontal stud. You can do the same for the bottom, but bend the tab out so far to find stable concrete or masonry to use concrete screws to attach. The mud bed the hearth will need will allow you to run that tab under the hearth to find good attachment there.