Views pager plugin with the option to have a different number of items on the first page.

VariablePager.php

<?php

namespace Drupal\views_first_page\Plugin\views\Pager;

use Drupal\Core\Form\FormStateInterface;
use Drupal\views\Plugin\views\pager\Full;

/**
 * @ViewsPager(
 *  id = "variable_pager",
 *  title = @Translation("Paged output, variable first page"),
 *  short_title = @Translation("Variable first page"),
 *  help = @Translation("Paged output with variable first page, Drupal style"),
 *  theme = "pager",
 *  register_theme = FALSE
 * )
 */
class VariablePager extends Full {

  public function getItemsPerPage() {
    $key = $this->getCurrentPage() <= 1 ? 'items_first_page' : 'items_per_page';
    return $this->options[$key] ?? 0;
  }

  public function defineOptions() {
    $options = parent::defineOptions();

    $options['items_first_page'] = ['default' => 10];

    return $options;
  }

  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
    parent::buildOptionsForm($form, $form_state);
    $form['items_first_page'] = [
      '#title' => t('Items on first page'),
      '#type' => 'number',
      '#description' => t('The number of items to show on the first page'),
      '#default_value' => $this->options['items_first_page'],
    ];
  }

  public function query() {
    parent::query();
    $limit = $this->getCurrentPage() < 1
      ? $this->options['items_first_page']
      : $this->options['items_per_page'];
    $this->view->query->setLimit($limit);

    if ($this->getCurrentPage() > 0) {
      $offset = $this->options['items_first_page'] + (($this->getCurrentPage() - 1) * $this->options['items_per_page']) + $this->options['offset'];
      $this->view->query->setOffset($offset);
    }

  }
}