Which brick for fireplace extension

[Dr Benz]

We want to create a smooth edge instead of the inverted steps that are there today. What would you recommend building this with? I'm guessing bricks are good, but there are different hardnesses, right?

The ones on the walls today are super hard.
Tips on similar ones.

Right now it's about making a straight line and plastering. In the long term, a cassette should be installed so it can be used.

Anything specific to consider here?

 

[Dr Benz]

For those interested, I installed regular fire-resistant bricks that were plastered in and turned out great. There are pictures in other of my threads.