My advice and experience is don't use Netcup. They are abysmal at customer service. I once registered a domain with them, and hadn't even paid, but then couldn't get my account deleted at all. Even multiple e-mails did not help and they insisted on keeping my data until half a year later or so. They absolutely behaved like complete holes, and I will never trust them again with anything. I don't know what they do with user data. Maybe they systems are just so terrible, that it is a huge effort for them to go and comply with GDPR deletion request and then they just don't do it.

Also their web interface doesn't allow you to delete your domain, even if you have not paid yet. So anyone could come and make some account and register a domain, but then not pay and they wouldn't remove it from their systems. The feel of their website is very antiquated and due to not being able to delete your domains, feels buggy.