I have modified several fireplaces using angle iron4”. The angle is about 3” longer than the opening. Cut 1-1/2” of 1 web on both ends. Move down to the mortar joint at the level or close to what you want. Cut the mortar out 1-1/2” deep. Slip the angle in with the cut web up and the lower longer web creating a shelf. Use refractory cement as a mortar and fill the joint where you cut out the mortar.
Now figure out how many layers so your staggers are correct as you will be starting at the bottom and working up.
Put down a layer of refractory cement followed by bricks ( butter the ends and bottoms with the cement, once the layer is in put a layer of cement on top of the layer and complete each layer the same as the first but with a stagger, the top corse of brick the hard part is filling the gap with refractory cement.
For additional strength the end brick if 1/2 can be removed but it looks like all your bricks are 1/2 so I would use caution trying this.
I have used this method many times, I have tried to use fire clay but had issues with cracking where refractory cement worked much better. The 4” angle iron provides the support for the span , on a narrow fireplace I have used 3” but prefer the 4” and you really can’t see it.
