i implemented the possibility to add a custom class set on the grid container (div.av-author-loop-container)
and implemented a css for that custom-class on loop-author.css to rule the column-count for all screen width :

/* ======================================================================
	OPTIONAL: AUTO-FIT GRID  in filter for automatic responsive columns
	Use    $atts['class']   = 'auto-fit-grid';   
   ====================================================================== */
#top.author .av-author-grid-container.auto-fit-grid {
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)) !important;
}

this means: Each column must have a minimum width of 250px — the entire width is then filled with columns based on this basis.

_______________

in list view – if you do not like to show the featured image in cover mode – then add to your quick css:

#top.author .av-author-list-image img {
  height: 100%; /* === unset - will bring the image to top === */
  object-fit: contain;  /* === contain - will show the whole image without crop === */
}

just for understanding – these are the default values from loop-author.php:

// Default arguments for Author Loop
$default_args = array(
    'type'              => 'grid',
    'columns'           => 4,
    'items'             => get_option('posts_per_page', 12), // WordPress setting, fallback: 12
    'contents'          => 'excerpt_read_more',
    'preview_mode'      => 'auto',
    'image_size'        => 'portfolio',
    'paginate'          => 'no',
    'class'             => '',
    'show_modified_date' => 'no',  // 'yes' or 'no' - shows "Updated: XX" if post was modified
    'orderby'           => 'date',  // 'date', 'modified', 'title', 'rand', 'comment_count', 'menu_order'
    'order'             => 'DESC',  // 'DESC' (newest first) or 'ASC' (oldest first)
    'fallback_image'    => 'placeholder',      //  'placeholder'  or URL to fallback image 
    'post_types'        => array('post'), // Array: array('post'), array('portfolio'), or array('post', 'portfolio')
);

// Content Options for 'contents' parameter:
    // 'title'                  - Title only
    // 'title_read_more'        - Title and Read More button
    // 'only_excerpt'           - Excerpt only (no title)
    // 'only_excerpt_read_more' - Excerpt and Read More button (no title)
    // 'excerpt'                - Title and Excerpt
    // 'excerpt_read_more'      - Title, Excerpt and Read More button (default)

if you like to have it this way (grid, 4columns, exerpt with readmore, only posts ordered by date – newest first) you do not need to set the filter.
Then only the second part is needed inside functions.php.

The filter could do more – you can even differ between authors – f.e by author name. :

// 1. Layout for different authors by ID
add_filter( 'avf_author_loop_args', function( $atts, $context ) {
    if( $context == 'author' ) {
        // Common settings for all author pages
        $atts['items']             = -1;
        $atts['image_size']        = 'gallery';
        $atts['contents']          = 'excerpt_read_more';
        $atts['post_types']         = array('post', 'portfolio'); 
		
	// Fallback-Image from Media Library (Attachment-ID)
        $fallback_id = 2052;  // Your image ID from Media Library
        $fallback_img = wp_get_attachment_image_src($fallback_id, $atts['image_size']);
        $atts['fallback_image'] = $fallback_img ? $fallback_img[0] : 'placeholder';
        
        $author_slug = get_query_var( 'author_name' );

        // Individual configuration for a specific author
        if( $author_slug == 'guenni007' ) {
            $atts['type']              = 'grid';
            $atts['columns']           = 3;
        } 
        else {
            // default setting for the rest
            $atts['type']              = 'list';
        }
    }
    return $atts;
}, 10, 2);

the fallback image i get from media-library with the attachment ID
Advantage of this solution – the fallback image is based on the selected image size setting.

see:
grid for guenni007: https://basis.webers-testseite.de/author/guenni007/
list for johndoe: https://basis.webers-testseite.de/author/johndoe/

now my final solution is – to upload 3 files to the child-theme includes folder:

/wp-content/themes/enfold-child/
└── includes/
  └── loop-author.php ← PHP Loop (loads the css once)
  └── loop-about-author.php
  └── loop-author.css ← all Styles (Grid + List)

loop-author.php: https://pastebin.com/yaXj5Zfw
loop-about.php: https://pastebin.com/QQ84LGaw
loop-author.css: https://pastebin.com/zTJMtth9

to style the author page now are two entries for child-theme functions.php

// 1. Configure layout and settings
add_filter( 'avf_author_loop_args', function( $atts, $context ) {
    if( $context == 'author' ) {
        $atts['type']              = 'list';               // 'grid' or 'list' Layout
        $atts['columns']           = 4;                    // columns count
        $atts['image_size']        = 'gallery';            //  image-size
        $atts['contents']          = 'excerpt_read_more';  // Content-Type
        $atts['items']             = -1;                   // Posts per page
        $atts['paginate']          = 'no';                // Pagination  on/off  (yes or no)
        $atts['show_modified_date'] = 'yes';               // 'yes' = shows  "Updated: XX"
        $atts['orderby']           = 'date';               // orderby : 'date', 'modified', 'title', 'rand', 'comment_count'
        $atts['order']             = 'DESC';               // order: 'DESC' (newest or Z-A) or 'ASC' (oldest or A-Z)
	$atts['fallback_image']     = get_stylesheet_directory_uri() . '/includes/fallback.jpg';  // an url or  'placeholder'
	$atts['post_types'] 		= array('post', 'portfolio');    	// 	count posts or portfolios or both
    }
    return $atts;
}, 10, 2);

// 2.  IMPORTANT: Customise query for pagination, sorting and post types
add_action( 'pre_get_posts', function( $query ) {
    if( ! is_admin() && $query->is_main_query() && is_author() ) {
        // Get the filtered arguments from above
        $default_args = array(
            'items'      => get_option('posts_per_page', 12),
            'orderby'    => 'date',
            'order'      => 'DESC',
            'post_types' => array('post')
        );
        $atts = apply_filters( 'avf_author_loop_args', $default_args, 'author' );
        
        // Set Query-Parameters
        $query->set( 'posts_per_page', $atts['items'] );
        $query->set( 'orderby', $atts['orderby'] );
        $query->set( 'order', $atts['order'] );
        
        // Set post types - IMPORTANT: even if there is only one type
        if( ! empty( $atts['post_types'] ) && is_array( $atts['post_types'] ) ) {
            // WordPress requires 'any' or an array with multiple types
            if( count( $atts['post_types'] ) == 1 ) {
                $query->set( 'post_type', $atts['post_types'][0] );
            } else {
                $query->set( 'post_type', $atts['post_types'] );
            }
        }
    }
}, 999 );

see result page : https://basis.webers-testseite.de/author/guenni007/
Unfortunately, no translation for But we are proud to say that %s contributed %s entries already. has been added to the (at least German) lang files yet. That is why it is in English on my example page.

i tested all 3 blog styles – it works on all of them.