Roman bread

[M. Demetrius]

Given that energy was expensive, I can't see the Romans letting anything sit in an oven longer than necessary

A wood-fired oven will hold heat for much longer than 2 hours. Once heated to baking temperature, regular breads requiring high heat could be baked first, followed by those that need less, ending with hardtack as the last batch. They'd simply watch and take the breads out at the required time. Hardtack baked in higher heat burns (not quite as much as the carbonized Pompeii bread, of course) before it's done in the middle.

Typically, there would be a stone floor in the oven. The stone and brick/clay surrounding surfaces heated up by a wood fire made for a very efficient oven.