Mass Email Functionality Improvement
The mass email functionality in Salesforce as it stands is not very robust. Using Flow to send mass emails with a loop achieves the same thing and can be triggered based on a field/ change of fields/ set.
The link below is a package I downloaded that lets you send HTML emails CC, BCC and add attachemnts, howver the only thing its not able to do as of now is is send to multiple CCs/BCCs by looking up and separating the emails with a comma like the "Send Email Flow Core Action's Email Addresses (comma-separated)" field.
If you are able to make minor edits the Apex code so that the "SendTOthisStringCollectionOfEmailAddresses" field in the SendHTMLEmail is able to function like the "Send Email Flow Core Action's Email Addresses (comma-separated)" field that is native to Salesforce it would be a massive improvement in the quality and functionality associated with sending emails from the systm.
Flow Core Action Send Email :https://help.salesforce.com/articleView?id=flow_ref_elements_actions_sendemail.htm&type=5