You can simply add waitForExpression('5s') to your request.

$request = \Gotenberg\Gotenberg::chromium('http://gotenberg:3000')
    ->pdf()
    ->waitForExpression('5s')
    ->url('https://my.url');

It's not in the documentation, but if you have PHP Intelephense, then it will hint you the available methods.

I'm using phpstorm's idea plugin.