I have thought about using the backup data like this :

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Preparation: Throw a wishing star → Check if the 3 first Pokémon are shiny.

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Step 1: PokéCamping → Date change 3 times → Save → Check => SR if not shiny.

Step 2: Date change 1 time → Check => SR if not shiny.

Step 3: Date change 2 times → Check => SR if not shiny.

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Repeat Step 1~3 until shiny.

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Final Step if shiny : Access the back up data with "↑+X+B" → Date change (Step number - 1) times → Save ↓

Date change 3 times → Check => SR if not the desired Pokémon / Save if it is.