^^^^ I also think that the AI image makes the stone look thicker than 1/2".

I would stop the jamb just shy of the front of the brick on the outside wall. The smooth quartz slab on the jamb should be intentionally stepped back about 1/4" (up to 1/2" depending on the mortar joints as described below) from the face of the white painted brick wall. This subtle reveal hides the natural waviness of the brick edge, avoids an awkward flush seam, and gives the shower opening a polished architectural frame.

Leaving an intentional small step back will help hide imperfections = brick faces are naturally uneven and wavy. If you try to make a perfectly straight, flat piece of quartz sit perfectly flush with wavy brick edges, you will end up with ugly, inconsistent gaps. Stepping the quartz back hides those variations.

I like the third image that shows a straight line across the top where the jamb meets the arch. The jamb should stopexactly where the arch begins to curve (= the spring line). Leave the curved portion of the arch as raw, exposed brick. This looks better to me than the way the first AI image has the jamb of the shower meet the arched area by using a small angle. However, whether that will work depends on the spring line = exactly how the jamb meets the arch in real life (if there is room for the jamb to sit under the arch before the curve).

I would not stop the jamb short of the arch (how will you determine where to stop it + how would the top be finished?)

In order to determine the exact width of the stone jamb:

1. Measure to the Inner Mortar Line: Have them measure the depth from your internal shower waterproofing out to the inner recess line where the brick turns the corner.
2. Subtract 1/4 Inch: Deduct about 1/4 inch from that depth measurement. When the flat slab is glued flat against the inner framing, it will naturally stop just shy of the front corner, creating a clean, professional shadow line. However, if your mortar line is much deeper than 1/4" - you may want to subtract a little bit more (but not more than 1/2" - 2-3" is too large of a reveal in my opinion - your AI images don't have a 2-3" reveal).

I tried to make an AI image of what I thought would look good - however, AI couldn't understand how to line up the jambs - instead of having a very small (intentional) reveal, it made the jambs the same width as the horizontal curb at the bottom (which should slightly overlap the brick + have a small tilt back into the shower). AI also added a small return on the outside of the shower jamb (the piece overlaps the outside edge just a bit). It does allow you to see why it would look better to leave 1/4" reveal (you won't have uneven shadows created due to the brick faces not ending in a perfect line).

However, it does show how the jambs run right up to the spring line:

It would look really nice if you could leave the exposed brick vs. adding a solid jamb - however, I know that there are issues created by doing that - especially if the brick is older. Also, adding a heavy glass door straight against the brick wouldn't be a good option (if you didn't need a glass door to contain the water = you could just leave the brick uncovered if the brick was prepped correctly + very solid).

You are going to have a beautiful bathroom - I am jealous!