@Count:
I really don't want Big Mom to be the one to try destroying Fishman Island. She seems like a stepping stone (albeit a big one) in the grand scheme of the plot outside of Elbaf, and I can't see her being dragged out as an antagonistic threat until that point. Especially when certain signs point to her crew never getting 1v1 fights with the Straw Hats. I would prefer it if Blackbeard or the World Government attacks Fishman Island. I can't see Kaido doing it because of Wano Country being so soon, even though they could show up briefly at the Reverie.
Mmm, well if Big Mom does decide to commit her resources and Fishman Island manages to make a stand, then she wouldn't necessarily destroy Fishman Island, but that could still be her downfall if the Government decides to help them and take a chance to attack Big Mom while she is trying to get revenge. As well as the fact that the Warlord system will be questioned and the Government might have a weapon to replace them which might be brought up during the Reverie.
But she should know that she can manipulate the Straw Hats by calling attention to Zeff's predicament. Either as a hostage or sending his decapitated head as an example of what she can do to other people that are dear to the Straw Hats. She might be more easygoing on the Vinsmokes at this point.
Luffy is out there now, so let's see if he can make it personal enough for her to only focus on them. If they do end up escaping, I imagine Luffy will make some sort of statement that will push the limits of Big Mom's ~patience~. Maybe even declaring that he stole the Poneglyphs and he will become pirate king and not her.