What Boon is better for Caeda?

Probably attack. Her attack stat is still fairly low.

Tbh her speed is fine; it’s her attack that’s nearly unusable. An atk boon will fix that

Yeah go with the +atk one, her Attack is the stat you need to dedicate the most resources to

Oh and probably obvious, but wasn't too sure from your description, make sure to merge them lol

I've had discussions with people who have Caeda+10 with +atk, and they tell me spd is never an issue if you have hone fliers, which suggests to me that one goad fliers will do it most of the time.