Pls try this code:

<script>
            function collapseAllGroups(name, level) {
                var allGroups = document.querySelectorAll("#" + name + " tr.dtrg-level-" + level);
                allGroups.forEach(group => {
                    var collapseIcon = group.querySelector("span.group-collapse");
                    if (collapseIcon.style.display !== 'none') collapseIcon.click();
                });
            }
</script>
        <?php
            DataTables::create(array(
                "name" => "myTable1",
                ...
                "onReady" => "function() {
                    myTable1.on( 'draw', function () { //use this datatables' name here
                        collapseAllGroups('myTable1', 0); //use this datatables' name here
                    } );
                    collapseAllGroups('myTable1', 0); //use this datatables' name here
                }",\
            )); 

You can change the group level value 0 to 1, 2 if you want to collapse the groups at those levels as well. Rgds,

Pls change the collapseAllGroups function a bit like this:

<script>
            function collapseAllGroups(name, level) {
                var allGroups = document.querySelectorAll("#" + name + " tr.dtrg-level-" + level);
                allGroups.forEach(group => {
                    var collapseIcon = group.querySelector("span.group-collapse");
                    if (collapseIcon && collapseIcon.style.display !== 'none') collapseIcon.click();
                });
            }
</script>

And the "onReady" event should be something like:

                "onReady" => "function() {
                    myTable1.on( 'draw', function () {
                        collapseAllGroups('myTable1', 2);
                        collapseAllGroups('myTable1', 1);
                        collapseAllGroups('myTable1', 0);
                    } );
                    collapseAllGroups('myTable1', 2);
                    collapseAllGroups('myTable1', 1);
                    collapseAllGroups('myTable1', 0);
                }",

Hope it works. Tks,