I'm part of the team working on Workplace Analytics and MyAnalytics, so I think I can help answer your question. Workplace Analytics and MyAnalytics are currently two separate applications, even though they have similar names and both measure signals from Microsoft Outlook and Teams. MyAnalytics does all its calculations right in your mailbox - the data doesn't "go" anywhere, so privacy is built-in. As you know, you can ask your organization to turn off MyAnalytics for you, and even if it's turned on, individual users can also turn off some or all of its features if they don't find them useful.
By contrast, Workplace Analytics processes data from lots of "mailboxes" (user accounts) so that trends can be understood in aggregate. Workplace Analytics has a number of privacy controls in place to protect the identities of any one mailbox that forms that aggregate data set, but the big one that controls "opting out" is managed by your organization's Office 365 administrator. When they assign Workplace Analytics licenses to a group of mailboxes, Workplace Analytics will processes data from those mailboxes (and no others).
If you would prefer that your data not be part of this larger aggregate data set, I would recommend reaching out to your IT administrator to request that they either not apply the Workplace Analytics license to your mailbox, or that they remove the Workplace Analytics license from your mailbox if it has already been applied. Once they do this, Workplace Analytics will no longer process your data.
I hope this answer helps!