develop Gmail addon for mail merge and schedule Google Sheet

We would like to have a custom build Gmail addon / Google sheet Addon to achieve mail merge and mail scheduling. It will work like following: The Google sheet will have 'Send to Email Address', 'Message ID', ''Schedule DateTime', 'Message status', 'FirstName', 'LastName', 'Column 3', 'Column 4' Another Google Sheet will have 'Message ID', 'Message Title', 'Message Body', the message title and message body can use some of the tags specified such as 'Hi {FirstName}' When a 'Scheduled Datetime' on the GoogleSheet is hit/meet the current datetime, the Message matching the message ID on that row will be sent, to the email in 'Email Address' column. In this method, the new message is constructed on the fly. When datetime is hit, create message and send message based on cell data on google sheet. If this is not possible, we can also consider using gmail draft message, but keep in mind the draft message will be sent repeatedly, so it needs to stay in draft even if a message has been sent using the message. The 'Message Status' is default 'Scheduled', will update to 'Sent on {sent datetime}' after the message is sent.