Why is my brioche dry?
As per Stephie's comment, I'm adding this as an answer, even though it has to be largely conjectural.

10g of yeast is a lot of yeast for this amount of dough. With a five hour proof at room temperature, I think it is very likely that your dough is overproofed. This means the gluten structure over-develops and then becomes exhausted. So rather than a matrix of small bubbles you get a few big cavities in an otherwise heavy loaf.