ismaels filter (ava_inside_main_menu) is a brilliant idea.

maybe it is easier to use if we insert a social bookmarks shortcode:
then we can use all those benefits of enfold options dialogs – and hover styling etc.

function social_bookmarks_shortcode_inside_main_menu() {
  $social_args = array('outside'=>'ul', 'inside'=>'li', 'append' => '');
  $social = avia_social_media_icons($social_args, false);
  return $social;
}
add_shortcode('social-bookmarks', 'social_bookmarks_shortcode_inside_main_menu');

add_action( 'ava_inside_main_menu', function() {
    echo do_shortcode('[social-bookmarks]');
});

Quick css for your setting (left burger – logo – socials )

#top div .logo {
  z-index: 101 !important;
}

#top .main_menu, 
#top .avia-menu {
  width: 100%;
}

#avia-menu #menu-item-search {
  position: absolute;
  right: 0;
  left: auto;
}

#top #wrap_all .av-hamburger-inner {
  display:block;
  top: 0;
  margin-top: 4px;
}

.html_header_top #top #wrap_all .av_logo_right #header_main .logo {
  position: relative;
  left: 50%;
  right: auto;
  margin-left: unset;
  margin-right: unset;
  transform: translateX(-50%);
  width: auto;
}

.responsive #top #wrap_all  .main_menu {
  width: 100%;
  display: flex !important;
  flex-flow: row nowrap;
  justify-content: space-between;
  height: inherit;
}

#top #wrap_all .menu-item-avia-special {
  display: block ;
}

.responsive #top #wrap_all #header .main_menu ul.social_bookmarks {
    display: flex;
}

#top #wrap_all .main_menu ul.social_bookmarks li a {
  height: 30px;
}

#top .main_menu .social_bookmarks li {
  width:40px;
  margin-left:5px
}

#top .main_menu .social_bookmarks li.avia-svg-icon img[is-svg-img="true"],
#top .main_menu .social_bookmarks li.avia-svg-icon svg:first-child {
  height:1.1em;
  width: auto;
  margin-top: 4px !important;
}

@media only screen and (max-width: 767px) {
  .responsive #top #wrap_all .container {
    width: 95%;
    max-width: 95%;
  }
  .html_header_top #top #wrap_all .av_logo_right #header_main .logo {
    max-width: 200px;
    transform: translateX(-57%);  /* === an individual correction - depends on how many social icons are present=== */
  }
}

see : https://elementar.webers-webdesign.de/

PS: for more than 3 social bookmarks it gets complicated on small mobile screens. So maybe in this case we set them to display none – or switch in this case to header meta solution

PPS: i tried first that option to show social icons via Header – Extra Elements – Header Social Icons : “display in main header area”
but this was to complicated to find the right css positioning options – especially on shrinking headers.

sorry that “Link to: ” comes from Enfold – i thought that you had inserted it to the post title

the link comes from enfold itself. You do not need to insert on title input field links.

you can see that if there are more than 1 post inside a category – it works:
https://kunst-en-verhalen.rhijnhof.nl/pieta/

what makes me wonder is that on your posts with category: oorlogsslachtoffers (only 1) – there should be no post-navigation. ( Do you have in the page title that p-tag ?)
so maybe Ismael could help you with this inside your installation

PS : the setting on Enfold > Blog Layout > Single Post Navigation setting is not important if you use the filter from : above

With the help of this filter, you can even handle post-types differently.

a function like this to remove the image-sizes enfold likes to have:

function remove_enfold_image_sizes() {
// do NOT remove widget size, is used in backend portfolio items!
// remove_image_size('widget');
  remove_image_size('square');
  remove_image_size('featured');
  remove_image_size('featured_large');
  remove_image_size('portfolio');
  remove_image_size('portfolio_small');
  remove_image_size('gallery');
  remove_image_size('magazine');
  remove_image_size('masonry');
  remove_image_size('entry_without_sidebar');
  remove_image_size('entry_with_sidebar');
  remove_image_size('shop_thumbnail');
  remove_image_size('shop_catalog');
  remove_image_size('shop_single'); 
  remove_image_size('shop_gallery_thumbnail');
}
add_action('init', 'remove_enfold_image_sizes');

i guess does only influence the calculation of newly uploaded images.
The former uploads are not influenced on that. So these sizes are still in your uploads folder. (have a look by ftp inspection if this is the fact)
the use of Force Regenerate Thumbnails – removes those already calculated image-sizes that are not registered.

btw.: https://github.com/KriesiMedia/enfold-library/blob/master/actions%20and%20filters/Layout/avf_post_nav_settings.php

with this snippet you do not even need to set the enfold options on post-navigation.
You can set with that filter even to loop:

function my_avf_post_nav_settings( array $settings ){
  if( true === $settings['is_hierarchical'] ){
    $settings['skip_output'] = true;
    return $settings;
  } 
  // the post-types inside the array are allowed to have post-navigation
  if( ! in_array( $settings['type'], array( 'post', 'portfolio' ) ) ){ 
    $settings['skip_output'] = true;
    return $settings;
  }
  $settings['loop_post_nav'] = true;
  $settings['same_category'] = true;
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false; 
  return $settings;
}
add_filter( 'avf_post_nav_settings', 'my_avf_post_nav_settings', 10, 1 );

btw. if you set this (is_hierarchical) condition to false – then even pages will have post-navigation