Following up - the "Approve Estimate" functionality does exist at the online estimate page, but this link is not included in the email. The workaround is to either:

FOR OTHERS READING THIS AND HAVING THE SAME ISSUE HERE IS A WORKAROUND:

1. Do not use the "Send" function from Quickbooks, but instead manually send a pdf AND link to the estimate to your potential clients.

Or

2. Use the "Send" function which will email a pdf and an ugly/unformatted email to your client, and follow up with a link to the online estimate where they can approve it.

To find the link to the online estimate use "SHARE ESTIMATE LINK" from the Sales --> Estimates view - see screenshot