If you have pre made templates on sendgrid then use below API methods to send those,
https://sendgrid.com/docs/API_Reference/Web_API_v3/Transactional_Templates/templates.html

If you want html builder for prepare the email content then, use HTML editor’s. and use the Sendgrid Api V# for sent the html content.

there are more available plugins for html editor’s at https://bubble.io/plugins?q=editor