Darkness will screw up the rogue. He will have both advantage (target can't see him) and disavantage (can't see the target) which cancel out, but the disavantage will stop his sneak attack.
More notably, you lack any AoE (rogues being extra bad at groups). So i'd suggest shatter. Not something to cast every battle, but great against a pack of kobolds. Upgrade to destructive wave at 9.
And consider some utility like fly, or dispell magic.