FrogReaver
The most respectful and polite poster ever
How so? If you have to spend a bonus action before making the attack, you can't really crit fish. It's not a bonus action after you hit. It's exactly like the smite spells.
The game went to lengths to avoid people using two spell slots in the same round, except for reactions.
Is it established that this hypothetical bonus action requires you use the extra damage on your next hit?