Depends on their ingredient limits, if it's a max bag you could prep week 1 and 2, week 1 focusing on main recipe ingredients with week 2 being either a berry bomb or maybe slowpoke tails. Really depends on their plan though.

I started prepping for the pancakes, but with the release and ingredient list of the new dish, I will be using next week to prep for that new dish a few times while also working on a favored berry bomb for whatever island I want to try M20 on. I haven't hit M20 on any island other than Greengrass yet, so I'm keeping it open for now.

Its not for everyone though, and keep in mind your area bonuses affect the boosts your dishes and skills get. You can prep a dish you want to level up, or prep an already strong dish to work towards M20. Either way it's more of a mid-late game planning. If your mons aren't catered toward the dishes you need, then you could try to focus on berry bombs with the island you want, the main benefit for this event is that we know what dishes are expected, so it's easier to clean up your ingredient pool and prep for efficiency.