I think the English sentence does not imply 准备. I feel, 好了吗 would suffice.
I feel like if you're going to be so strict on other questions, you might as well add a "ni" here for the "you". Otherwise it could just be "ready?" or "done preparing?"
Context, context. Even though 你 is not explicitly used here, it is common to say this in conversation as it refers to another person other than the speaker.
Can i say "你准备了吗"？ or 好 is important here?
你准备了嗎is like saying,"did you prepare?"
我准备了= i have prepared, 我准备好了= i am done preparing, imply => i am ready