Rather than supporting a proprietary product like sendy I would recommend using the far superior civicrm which is fully open source, has a great community around it (not used by marketers and seo-spammer-types but rather by ngo's)and is totally extensible to grow with your needs. It is simple to be GDPR compliant by default and is really easy to get started. It obviously has a far bigger scope but you don't need to take advantage of all that additional functionality.