How can I require at least one checkbox to be checked

to go this way via filter would not be necessary here, but I just left it like that for my Contact Form 7 form. I can assign a class to these acceptance checkboxes, and I can use this class to address several boxes; and only if all of them are checked, the Send button is released.

by the way normal transition on submit button is 0.3s ease-in-out
you can have it in this case a little different by:

.avia_ajax_form p input[type="submit"] {
  -webkit-transition: all 1s linear;
  transition: 		all 1s linear;
}

I changed the code a little bit above. So that you don’t have only one Acceptance checkbox – assign a class to the IDs – here I have chosen acceptance.
You have to be careful when assigning multiple elements – they are separated by commas but are enclosed in quotes as group.
$('#avia_5_1 , #avia_6_1').addClass('acceptance');
all your acceptance checkboxes must be listed here

now you can also see the advantage over .filter : Only if all elements with the class pass the filter, the statement is considered true.
see again: https://webers-testseite.de/contact/

function change_submit_button(){
if(is_page('123456')){
?>
<script type="text/javascript">
(function($){
$(document).ready(function(){ 
	$('#avia_5_1 , #avia_6_1').addClass('acceptance');
	$(".acceptance").prop('checked', false);
	$('form.avia_ajax_form').find('input[type="submit"]').val("Stop").css({
			'background-color': 'red',
			'pointer-events': 'none',
			});

	$(".acceptance").on( "click", function(){
	    var a = $(".acceptance" );
	    if(a.length == a.filter(":checked").length){
	        $('form.avia_ajax_form').find('input[type="submit"]').val("Submit").css({
			'background-color': 'green',
			'pointer-events': 'auto',
			});
		}
		else{
			$('form.avia_ajax_form').find('input[type="submit"]').val("Stop").css({
			'background-color': 'red',
			'pointer-events': 'none',
			});
		}
	});
});
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'change_submit_button');