You go Thief Rogue 3 for Athletics Expertise, Cunning Action, and Fast Hands.

The extra bonus action is super handy when paired with GWM, Reckless Attack, and Rage. Your first round is: pop Rage, run up and Reckless Attack, and hopefully you crit (happens ~2/3rds of the time, almost every time if Hasted) for the bonus action attack. Berzerker insures you have that BA attack every round, but you get one so often from GWM that I don't really bother so I would actually recommend Wildheart or Wild Magic instead.

Not procing SA damage doesn't even matter since the build generally puts out so much damage (80-160/round at lvl 8) that nothing in their vicinity really survives.