Hi,
a) I think yes, Easy Profile extends Joomla user management (not replace), so registration form is the same of Joomla with many improvements.
Anyway you can already make your registration form GDPR compliance with Easy Profile, you need only to add correct privacy policy and a checkbox (like this site). an example here
https://www.easy-profile.com/support/custom-link-in-core-user-fields.html
About GDPR, we have also prepared a plugin to allow to user to delete own account, if you want to try it then see this topic
https://www.easy-profile.com/support/gdpr-compliance.html#reply-21673.
This plugin is not in the download area because probably Joomla 3.9 will implement a own privacy solution and APIs to give to the developers the ability to make extension GDPR compliant in a centralized and automated way.
b) Yes, also Acymailing allow you to have custom fields in registration form. you can find them at Components->Acymailing->Configuration