It's a pity DataTables hasn't had this option yet but we could work around it with js function like this:

DataTables::create(array(
        "name" => "rowDetailTable", // must have a name to be used below
        ...
        "rowDetailData" => ...,
        "onReady" => "function() {
            document.querySelectorAll('#rowDetailTable_wrapper .fa-plus-square')
            .forEach(function(expandIcon) {
                expandIcon.click();
            }); // expand all row details
            rowDetailTable.column(0).visible(false); // hide the expand/collapse detail column
        }"
    )); 

Let us know if it works for you or not. Tks,