Many folks do brisket for the entire cook above 300, some as much as 350. You have NOT ruined anything here.
If the bark is where you want it, foil it and cook it on through. If you want it darker, wait a bit.
Basically, there's nothing to say that you cannot get a juicy piece of meat just because the cooker temp got to 300.
Big JT's Smokin' BBQ Competition Team
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