population category dropdown and sub category onselect

I want to display categories and if the category is selected then display parent’s sub categories… I know there is some thread about this question but none of them fit to my problem.

I did 2 functions:

—- one for get all categories —-

    function get_cats() {
    
            $args = array('posts_per_page' => '-1',
                'post_type' => 'cust_categories',
                'post_status' => 'publish',
                'suppress_filters' => false
            );
    
            $cust_query = get_posts($args);
    
            if (!empty($cust_query)) {
                $counter = 0;
                foreach ($cust_query as $key => $dir) {
                    $selected = '';
                    if (intval($dir->ID) === intval($current)) {
                        $selected = 'selected';
                    }
    
                     echo '<option ' . $selected . ' value="' . $dir->ID . '">' . get_the_title($dir->ID) . '</option>';
                }
            }
    
    
}

—- one for get all sub categories —

function get_subcat($category) {
    $parent_category_ID = $category->term_id;
    $args = array(
        'hierarchical' => 1,
        'show_option_none' => '',
        'hide_empty' => 0,
        'parent' => $parent_category_ID,
        'taxonomy' => 'sub_category'
    );
    $subcategories = get_categories($args);
    foreach ($subcategories as $subcategory) {
        $link = get_term_link( $subcategory->slug, $subcategory->taxonomy );
        echo '<option value="'.$link.'">'.$subcategory->name.'</option>';
    }
}

And I did a form which displays them in a dropdown:

<div class="form">
<label for="category" class="formlabel">Category</label>
<div class="fieldcontainer">
<select name="category" id="category">
<?php echo get_cats(); ?>
</select>
</div>
</div>
                            
<div class="form">
<label for="sub_category" class="formlabel">Sub Category</label>
<div class="fieldcontainer">
<select name="sub_category" id="sub_category">
<option value="">Select</option>
<?php echo get_subcat('sp_categories'); ?>
</select>
</div>
</div>

Everything works fine but I want to display the selected category’s (parent’s) sub category only in the sub category dropdown, how can I achieve this?