After the event ended, we saw how Firebrand immediately arrived in the region, certain that the adventurers had taken something that was his property.

With this same certainty, he should immediately attack the players who triggered the event. He looks directly at the group and roars, attacking would be the logical sequence of this.

And not just stand there eating grass.

So only after this initial attack against whoever triggered the event, should he enter "standby mode".
However, instead of remaining static, he could walk or fly around the arena, while roaring as if he were actually searching for something.
Then as soon as a new group of players entered the area, he would immediately attack those players, thus starting a new battle.

Another suggestion I would make is to increase the Boss's aggro area. Because, although on the minimap it seems like a large area, we can see Steven and his group getting very close to the dragon before it actually enters "combat mode". Which doesn't make sense for a dragon that is enraged and looking for something.