Using Sales > Invoices as mentioned above will not work. Yes, you get all your invoices.

How do you filter all the invoices you have to just those you marked "send later"?

The correct way to do this is go to Sales > All Sales tab

Click Filter

Type: Invoices

Status: All Statuses

Delivery Method: Send Later

Apply

Then it only shows you the invoices you have not sent.

Click the top checkbox to select all, Batch Options > Send Transactions

After they are sent, the flag of "send later" is removed.