Re: "opt-out was probably the best choice"
Afraid the lawyers would have much of a chance. Repeat after me:
GDPR ONLY AFFECTS PERSONALLY IDENTIFIABLE INFORMATION.
GDPR ONLY AFFECTS PERSONALLY IDENTIFIABLE INFORMATION.
This data is anonymised, no compliance required.
Doesn't matter if it's an opt-in or an opt-out. It's anonymous data.
(UK spelling)