Hello, Mark, and welcome to the BrickWood forum.
Sounds like a beautiful way to finish your hearth…and in my opinion it’s not a huge deal at all. Pizzas go in on a peel, and they come out on a peel. There’s no scenario where food or pans or anything else is going to be stopped by the lip.
For someone who couldn’t live with the idea that they have a ¼ inch or ⅜ inch “step down” to the hearth, the fix would be to make your hearth stand up proud to the brick frame by that amount, and allow the base course of mortar to level it up for the arch and back wall. For that increment I don’t believe it would introduce any stress worth worrying about. If you went that route you would want to determine the thickness of your ceramic tile plus whatever adhesive you’re using in advance. Also, there is a slight chance that you’d “miss” by a little bit, but it’s much closer.
I would for sure not want the hearth firebrick to stand any higher than that small increment.
Hope this helps, and looking forward to hearing more about your build!