Both need maxing. If you max only the base hero and the second costume, you will get the same costume bonus as before when maxing the only costume.
Basically, costume bonus now have 6/8 tiers (for 3* and 4/5*), each ascension of a costume (except the last one that you need to reach max level + 8/8 SS level) raise the costume bonus one tier for this hero and their costume copies.
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