Small Outbox is a simple yet efficient mail merge Google Sheets Add-ons.  This is absolutely free and only limited to Google Apps Script quota.  There is no fancy UI, all configurations are done in the cells in the sheet.  The current version will run through entire list and send 10 each time until either it finishes or hits any limit.

To leverage the power of Google Apps, the add-on simply use Gmail draft as template, so you can personalize the best email template using existing tool with minimal learning curve.  To insert merge fields, simply place {{field name}} in the email where field name is exactly the same as column headings in your spreadsheet and the merge will be done automatically.  e.g. if column name is First Name, then field in email will be {{First Name}}.

For more information, please do not hesitate to visit the product website at:
https://sites.google.com/quickfixters.com/add-onsuniverse/small-outbox-mail-merge