Rather than include something in the text of the documents to mark the Preamble, wouldn’t it be simpler just to create a custom metadata item in Project > Project Settings > Custom metadata?
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If you make it a checkbox, then you can simply tick it on or off for specific documents (using in the Outliner / Inspector / Binder etc).
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The new metadata item will appear in the Project Search dropdown, so you can create a Collection as before (just tick Preamble in the dropdown box and then have ‘True’ in the search field.
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If you save the search as Preamble, then it will become a dynamic collection, and you don’t have to worry about adding or removing text from documents, or adding them manually to the collection: all you need to do it tick or until the Preamble checkbox and the membership of the Preamble Collection will change automatically.
I mention this not because your solution doesn’t work, but because custom metadata is intended to be used in this sort of scenario (amongst many others…), so it’s a more general and flexible solution.
HTH.