I have not tried the carbon, but have owned a number of ovens, including my favorite, the Blackstone and the Koda 16. IMO, the Koda 16 is a nice choice for 12 inch pie. I have posted here a few times that I think it is not ideal for much larger pies, the heat differential between the left rear corner and the right front is extreme, and makes it challenging to get an evenly cooked pie, since it is time to turn before the front sets. Some posters have posted photos of some nice pies, though I think they were in the 12 to at most 13 inch range. I saw the Breville at a demonstration, and would have bought it if it could cook a larger pie, but the upper end on that is 12 inches, and I prefer a bigger pie.
To me, the Breville would be the clear winner of the two for 12 inch pies, because if it is raining or cold outside, it would be much more pleasant to use than an outdoor oven. Nearly every photo I have seen of a pie coming out of that oven is perfectly cooked. Also, it has an element under the stone, which means it should recover pretty quickly. The Ooni does not have any under stone heat, so the only way to recharge the stone is when you have removed the first pie. I haven't tested how quickly it will recharge, but my guess is that the Breville will be quicker.
I have never seen the Carbon, so can't comment, but a number of people like it.
In some ways it is a good time to be looking for an oven, there are lots of choices. Even more are being released, the Halo Versa 16 looks pretty interesting, its price point is very attractive, it says it has a rotating stone, and an infrared element heating from below, together with a flame from above
https://halo-pg.com/versa-16-pizza-oven/ though it is not clear whether you can adjust the lower infrared flame . The downside is there is no way to know when it will come out. The Ooni Karu 16 seems to have learned that the burner layout in the Koda 16 is not ideal, and the door looks pretty cool. That may be a better challenge to the Breville if you want to go with larger than 12 inch pies, though again, it doesn't have a burner under the stone like the Carbon, and it is pretty expensive.