[simonblack]

3. How to completely delete all files from Dropbox and permanently rescind their ability to access my data in the future.

Good Luck with number 3!

This is akin to posting your bank details on a local wall. You can put it out there, but there's no certainty that you can ever withdraw it. Your best hope is that Dropbox is running low on storage and needs to reuse some disk space that has your data on it. That may happen sooner rather than later but don't hold your breath waiting for it to happen.

Maintain your own backup storage yourself. That's the only way to keep control of your own data.

By the way, who's doing the encrypting? You or the Dropbox alternative? If it's not you doing the encrypting, you may as well leave your files unencrypted. "If more than one person knows a secret, it's not a secret!".