I really think the mistake is seeing mimicry as a limitation, rather than a cool flavor. I've heard of DMs insisting that kenku can only use phrases heard _that_ session. Bad form. Kenku speak using phrases they've already heard. That shouldn't be a super hard limit because by the time someone is even just a teen, they've heard thousands of phrases. Do you think they hatch from the egg and wear earmuff the first X years of life? Do you think kenku don't talk to each other? I bet they pass down phrases from mother to child, friend to friend, master to apprentice. An uncle has a great joke that he heard a halfling tell, and now everyone in the family can do it perfectly. The mother kenku's ability to scold has been in the family a hundred years, supposedly from phrases originally heard form a green hag. Highly specific phrases may be a pinch, but you can be creative with that. Otherwise, they can same most anything because most anything has probably been said around them at one time or another. How, not if, they say it, that's where mimicry should come in. Do you say "no" like an angry orc, or like a stern Nanny. Do you have a collection of 20 "no" voices that you use to fit the situation. Mimicry should be an opportunity, not a shackle.