When I tested, RATK changes are calculated at the time of the attack hitting, rather than at the time of use. You can even pre-shoot Chase Arrow then pop your Cake and the Cake effect applies when it hits.
Multipliers I honestly think apply at the time of hitting as well, considering I was using Brave Stance - How would the game know I was in front when Gwana hasn't even spawned yet? Although back at the time of testing I didn't have Rapid Shoot Mastery so I really should re-test multipliers.
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