I see very little point in getting just one level of Warlock. You lose a feat this way. That feat could be Spell Sniper, or Magic Initiate: Warlock. Both can give you access to the Cantrip already.
Two levels however give the Charisma damage bonus, and as Bard you can get to stupid levels of Charisma.
And yes level 11 Bard gives you up to two level 6 spells, one regular and one if you trade in another spell. Thats really your choice.
