Hi, Privacy Shield on its own doesn't guarantee they comply with GDPR. You can find more in my blog post: https://www.hubuco.com/blog/email-verification-gdpr
By the way HuBuCo is 100% compliant with GDPR
I use this email verifier https://winpure.com/ Quickly, conveniently, reliably.
It seems like you are forgetting a major player Zerobounce. It has U.S.- E.U. and Swiss-U.S. certified shield.