Solved Search form to filter 4 lists - any alternatives to URL filtering?

I'm trying to filter four lists using one search form but the "traditional" way of using a content plugin in an article with URL filtering is not working consistently for me, despite several days of trying and lots of help from luminaries in this forum. So I'm looking for an alternative.

In case it's relevant: The four lists are all copies of one main list which has multiple repeating groups. Each of the four lists focuses on one of these groups. Each of the four lists contain an element which is a databasejoin to a separate list containing names. The search form has a single databasejoin element connected to this list of names.

Is it possible to use some PHP code in the search form to filter each list at the same time?
Or some Javascript to do the same?
Or, and this might be a very silly question, can prefilters accept variables in the value field? And the search form creates that variable every time it is used??
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