.. selecting a column to set it to be conditional doesn't work ..
I take it you tried to 'hide' the entire column? Correct — that simply won't work. Conditional text works on text only.
Your GREP is fine (*), but you should take care when adding the index entry. If the index entry is inside the condition, it will also be hidden at the moment you generate the index …
(*) Except you probably also want to find the hyphen. In that case, use something like “^([w -]+)t”.
Word — of all possibilities! — supports a great feature for indexing. You can feed it a word list, à la index brutal, to generate what's known as a concordance list. That is a 'dumb' list, not copy-edited, just listing each and every word found. (Generating an index programmatically and having a good index are two separate things.) In Word, you can set up the input file as a two-column table; in the first column, you would enter the term to search for (in your case, “cookie star-shaped”), and in the second column, you enter the exact term as it should appear in the index (in your case, “star-shaped cookie”). I use this to generate index markers for proper names: “Einstein” in the left, “Einstein, Albert (1879-1955)” in the right column.
I suppose you could ask Marc Autret if he could add something similar to index brutal.
Well, well. As usual, Marc already foresaw that! Read how to do this on his web page: “ENTRY LINES SYNTAX : SPECIAL OPERATOR” (https://marcautret.free.fr/geek…..rut/en.php)