Later today I will show you how I have implemented the result. Basically it is the integration of the “logo” into the calculation routine of the shrink script.
We will add your Logo not by having a menu list point in the top-menu but only by placing it via child-theme functions.php and absolute positioning.
And this will be the only logo you need for that.
On your page you beginn the logo height at 300px – and it should end at 66px – so shrink-factor is 4.545
you can see here what we need to obtain that in the avia-snippet-sticky-header.js : https://kriesi.at/support/topic/shrinking-of-header-amount-an-info/#post-1044887
With that shrinkfactor we can calculate the two values to influence in the shrink script. The only the thing to do in this script will be the integration of the new “toplogo” to that calculation. No panic i will send you the edited script-file: https://pastebin.com/gi40An1c
Download-link: https://pastebin.com/dl/gi40An1c

But for now enough – detailed info later! IF – you are interested in that solution

do you have a svg on your logo ?
you can see here a playground with your logo – but if we had a svg we could set single parts of the logo to not be seen when the header is scrolled.
https://webers-testseite.de/cynthia/tuddal/

i only have styled it yet for the wider screens – but when you got the svg this comes next.

on the link of the masonry items – you have a class: portfolio_entries-xyz the xyz is the portfolio “category” ( this is actually a taxonomy)

you can place it in a pseudo-container like:
see here: https://webers-testseite.de/pureinstall/portfolio-masonry/
(this was originally a help to place the selection next to the masonry.)

.portfolio_entries-buecher .av-masonry-outerimage-container:before {
  content:"";
  width: 100%;
  height: 100%;
  position: absolute;
  background-image: url(https://webers-testseite.de/pureinstall/wp-content/uploads/Buecher.png);
  background-repeat: no-repeat;
  background-size: 100px;
  background-position: center 30%;
  z-index: 2;
}

and: on posts and categories the class is: category-xyz the category name is the xyz

for background-attachment : fixed
there is a better way to have it – because on IOS Devices – this looks strange – on scrolling it flickers
( google on that background-attachment : fixed and Safari Browsers )

there is a little trick to have that: the background-image that should have background-attachment : fixed goes to the pseudo container :before –
and that container is set to position fixed!
So the code above will look like this ( for my page-id ) :

#top.page-id-38479 {
  background-image: url(https://webers-testseite.de/wp-content/uploads/IMG_bg.jpg);
  background-repeat: repeat;
  background-position: center center;
  background-attachment: scroll;
}

#top:before {
  background-image: url(https://webers-testseite.de/wp-content/uploads/IMG_background_treatments.png);
  background-repeat: repeat-y;
  background-position: left top;
  content: "";
  position: fixed;
  width: 100%;
  height: 100%;
  -webkit-clip-path: inset(0 0 0 0);
          clip-path: inset(0 0 0 0);
}

#top.page-id-38479 #main {
  background-color: rgba(255,255,255,0.2) !important;
}

#top.page-id-38479 .main_color {
  background-color: transparent !important;
}

.html_stretched #top.page-id-38479  #wrap_all {
  background-color: transparent !important;
}

with the fixed image on repeat-y now

First of all – what kind of layout do you have choosen on Enfold: General Layout : a stretched or a boxed or at the end a fixed frame layout?
If you have a boxed layout f.e. – you will have then on General Styling a “Body Background” Option

But this is only one part of your example page.
If you like to have a full transparent content – like on your example page there had to be a lot of css settings to be made.

best would be if you start with that boxed layout – it is possible to have this with a fixed frame.
It is hard to have this with a slideshow in the background..
That was my old attempt to reach this – but the slideshow timing will not be synchronized the more time passes:
https://webers-testseite.de/goldfinger/
( These are two identical slideshows that runs on theory synchronized – the one on top shows only a narrow strip and has a high z-index – the other has a low z-index)
But that is not necessary for your solution with a static background-image.
It will be best to help you if we can see the page it concerns.
it is easy to have a body background-image via quick css f.e. and even to have more than one background-image on the same container:
you see on that page how it is done for a 1/1 container with three background-images and different settings :
https://webers-testseite.de/multiple-background-images/

in other words it is possible to have this http://naturkosmetik-studio.at/Images/IMG_background.png as one background pattern that is set to repeat or not to scroll away – and those leaves too but with no repeat.

maybe you have a /products etc in your bin!
empty that bin and try again

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');

and i guess you have some troubles with : CORS: https://kriesi.at/documentation/enfold/icon/#icons-are-showing-as-rectangular-boxes-

on my test page is stayes at the menu.
can you try this:

function shift_search_icon(){
?>
<script type="text/javascript">
(function($){
	$('#avia-menu').find('#menu-item-search').clone().appendTo('#menu-menu3');
         $('#avia-menu').find('#menu-item-search').remove();
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'shift_search_icon');

just to see if detach() is the problem

ok – we posted the same time.
How did you create that footer? is it a page as footer ?

as far as i know
1) the enfold main menu is always: .main_menu
( a class here because if you got menu below logo there will be an extra container ( #header_main_alternate) for that cloned menu)
2) the top menu : #avia2-menu
3) footer menu: #avia3-menu
these are theme_locations not names of the menu.

are you familiar with developer tools of your browser? Inspect the footer menu and see what ID this menu has got.

but nevertheless – have you seen my concerns about the position?

You like to have an acceptance button – but for enfold contact form?

By the way – dear Dev’s – might be a nice to have thing: acceptance Checkbox

You had to know the ID of the checkbox you like to have as an acceptance button. ( on my example it is checkbox with ID: avia_5_1 )
if you are not familiar with developer tools – you even can count from top to bottom – left to right. – you see on my test page that it is the 5th element on that form.
By the way – if it is the last visible element before submit you can use as selector even the class: av-last-visible-form-element
With the setting of that checkbox you can style and influence the behavior of the submit button:

this code here is for a specific page – you had to adjust it to your page ID.

code comes to child-theme functions.php:

function change_submit_button(){
if(is_page('38472')){
?>
<script type="text/javascript">
(function($){
$(document).ready(function(){ 
	$('form.avia_ajax_form').find('input[type="submit"]').val("Stop").css({
			'background-color': 'red',
			'pointer-events': 'none',
			});

	$("#avia_5_1").on( "click", function(){
	    var a = $("#avia_5_1");
	    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');

PS: Change value ( Stop, Submit ) to your needs.
results page: https://webers-testseite.de/contact/

background-color : none    /*** is not a valid value  ***/

it could not be the browser cache – because i see the header fixed too.
And any other cache? we had to find then that rule concerning to a fixed header – but even with the css set the header to postion absolute – it has to scroll away.

.html_header_top.html_header_transparency #header {
    position: absolute;
}

and there is the correct class on html: html_header_sticky_disabled
( with a sticky header there should be : html_header_sticky )

so this is strange!

as participant i do not see your private content – but lightbox has in its markup a mfp-bg ( background) and the mfp-wrap for the content – i guess what you look for is the mfp-bg opacity which is on default at 0.8
this is standard definition:

.mfp-bg {
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 1042;
  overflow: hidden;
  position: fixed;
  background: #000;
  opacity: 0.8; 
}

but sometimes ( depends on the source you open it must have a more specific selector like:

.mfp-zoom-in.mfp-ready.mfp-bg,
.mfp-zoom-in.mfp-ready .mfp-preloader {
  opacity: 0.8;
}