Search results always returns "No results" no matter what

Question

pre_get_posts hook that never works

I used the following code both in functions.php of my theme and in my custom plugin in the "after_theme_setup" hook (pre_get_post code only as it is alot to put in one question at once already)

/* The pre_get_posts code: */

add_action('pre_get_posts', 'ipgh_pre_get_posts', 10);
function ipgh_pre_get_posts( $query ) {
    if(is_admn() || ! $query->is_search() || ! $query->is_main_query() ) return;
    $keyword = $query->get('s');

    /* Just a test and doesn't even work either */
    $query->set('post_type', 'product_variation');

    /* My workaround, atleast I thought it would work: */
    $custom_post_types = get_posts([
        'post_type' 'product_variation',
        'post_status' => 'publish',
        'fields' => 'ids',
        /* Optional, I tried with and without it as it internally does change the SQL structure */
        's' => $keyword,
        'meta_query' => [
            'relation' => 'OR',
            [
                'key' => '_sku',
                'value' => $keyword,
                'compare' => '='
            ],
            [
                'key' => 'brand',
                'value' => $keyword,
                'compare' => 'LIKE'
            ],
        ]
    ]);

    /* This runs smoothly and indeed returns what I want. The IDs of the posts
       ( which are children of another post of type "product" but that shouldn't
       matter as they are all posts anyway, plus I've searched for child posts
       by their title and that works natively in Wordpress
       so it should work either way )
    */
    var_dump( $custom_post_types );

    /* This is where it just doesn't do anything, if you dump the query
       it seems that the query wasn't even changed at all
    */
    $query->set( 'post__in', $custom_post_types );

    /* Even tried this but ofcourse, doesn't work
       Note: I tried with both post__in and post_parent__in
       and each of these options seperately. Also tried to get
       their parent IDs and then pass them to post_parent__in
       but doesn't work.
    */
    $query->set( 'post_parent__in', $custom_post_types );

}

QUESTION: I want to be able to search with meta data AND post title like if theres no meta found then just the posts by title. But not matter how I change the query inside the pre_get_posts the frontend shows "no results" no matter what. Even if I change only one simple query_var.

Is there a way to just force the search in such way that I either add SQL myself or edit the template to force my own query or is there some hook where I can just return the plain posts that I need?