How to add an alt tag to the logo?

for the menu links : maybe you set for all links the content of the menu as alt attribute
this comes as the code above to child-theme functions.php:

function set_menu_text_as_alt(){
?>
<script>
(function($){
		$('#avia-menu .menu-item a').each(function(){
			var menuName = $(this).find('>.avia-menu-text').text();
			$(this).attr('alt', menuName);
		});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'set_menu_text_as_alt');

try first without $(document).ready(function() this must work too

for the headings you can do it – like the function above and set the content as the alt tag.
But that might be too long.

you can use my edited heading.php – if there is a link on the heading there will be an input field for alt attribute:

https://webers-testseite.de/edited-enfold-alb-elements/

https://kriesi.at/documentation/enfold/child-theme/

On default there is no child-theme present. But you can download a predefined one from link above.
A lot of those snippets here on board only work good with a child-theme.
You can update the parent theme without the risk of lost the made adjustments to the theme.

If you are not familiar with child-theme – read that carefully – and read my tutorial on that linked page above carefully too.