There’s no need to use a plugin if you use legacy templates on Sendgrid. This should help:
[Beginners Guide] to Sending Emails With Sendgrid - Work in Progress! - Tips - Bubble Forum
When you create a new template a default body HTML tag will appear on the page. Build your template around this and don’t delete. Save.
Then add the template ID in to Bubble’s settings. The body configured in the workflow will appear where the html tag is in the email template.