Create one Task for an Account when emails have been sent to all Contacts under the Account

Can't be done out-of-the-box.

Also note you have a race condition in your proposal: at the time you're checking if a Contact represents the last one awaiting an email reachout, other Contacts could be added to an Account, making the Account-Wide Alert Task immediately obsolete.

One custom approach that comes to mind: let all tasks be created, flip a Was Emailed flag on the contact, use Apex trigger to delete the Task before it comes to anybody's attention unless the Account has been fully emailed.