YAMMPreparation prepares the line to send mail via YAMM (Yet Another Mail Merge) #gas

Code.gs

function onOpen() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('YAMM Preparation')
  .addItem('Preparation', 'preparation')
  .addItem('Set folder of Details', 'setFolderOfDetails')
  .addToUi();
}

function preparation(){
  try{
    var new_data = [];
    var active_sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Email Sender');
    
    var data = active_sheet.getDataRange().getValues().map(function(r){return r[0]});
    var folderOfDetailsID = PropertiesService.getScriptProperties().getProperty('FolderOfDetailsID');
    var folder = DriveApp.getFolderById(folderOfDetailsID);
    var files = folder.getFilesByType(MimeType.GOOGLE_SHEETS);
    while (files.hasNext()) {
      var file = files.next();
      if(data.indexOf(file.getId()) == -1){
        var sheet = SpreadsheetApp.open(file);
        var email = sheet.getSheetByName('Email').getDataRange().getValues().toObjectVertical();
        var details = sheet.getSheetByName('Details').getDataRange().getValues().toHTMLTable();
        new_data.push([file.getId(), email['Email Address'] || '', email['Subject'] || file.getName(), email['Header'] || '', email['Footer'] || '', email['Attachment'] || '', details  || '']);
      }
      Utilities.sleep(99);
    }
    if(new_data[0]){
      active_sheet.getRange(active_sheet.getLastRow() + 1, 1, new_data.length, new_data[0].length).setValues(new_data);
      SpreadsheetApp.flush();
    }
  }catch(e){
    Logger.log(JSON.stringify(e));
  }
}

function setFolderOfDetails(){
  var folderOfDetailsID = PropertiesService.getScriptProperties().getProperty('FolderOfDetailsID');
  if(folderOfDetailsID){
    folderOfDetailsID = 'Now selected ' + folderOfDetailsID;
  }else{
    folderOfDetailsID = 'Please, enter ID';
  }
  var ui = SpreadsheetApp.getUi();
  var prompt = ui.prompt('Set folder of Details', folderOfDetailsID, ui.ButtonSet.OK_CANCEL);
  if(prompt.getSelectedButton() == ui.Button.OK){
    PropertiesService.getScriptProperties().setProperty('FolderOfDetailsID', prompt.getResponseText());
  }
}

DRAFT of Gmail

<<Subject>>

<<Header>>

<<Data>>

<<Footer>>

<<Attachment>>

Proto.gs

Array.prototype.toHTMLTable = function(){
  var table = '<table>';
  for(var i = 0; i < this.length; i++){
    table += '<tr><td style="border-bottom:1pt solid gray;vertical-align:middle;text-align:center;">';
    table += this[i].join('</td><td style="border-bottom:1pt solid gray;vertical-align:middle;text-align:center;">');
    table += '</td></tr>';
  }
  table += '</table>';
  return table;
}

Array.prototype.toObjectVertical = function(){
  var obj = {};
  for(var i = 0; i < this.length; i++){
    obj[this[i][0]] = this[i][1];
  }
  return obj;
}

Yet Another Mail Merge