Hello,
One small hack to automatically check all fields in Multi-Update modals.
add the following in Client Scripts -> Table-Specific -> Multi-Update Page -> Startup Script:
$("#u").trigger("click");
// hide select all row
$("#u").parent().hide();
// hide each field checkbox
$('input[type=checkbox][name^="u_"]').hide();
$('label[for^="u_"]').removeClass("custom-control-label");