deep-copy-folders.gs

// In case of "Exceeded maximum execution time" retry after some time
// Copied files and folders will not be copied again

// Changes:
// - Don't "search" for source and target folders
// - Inline some variables (to make it run faster)
// - Avoid Logger when possible (it can make script slower)

function start() {
  var source = DriveApp.getFolderById('get-source-folder-id-in-link');
  var target = DriveApp.getFolderById('get-target-folder-id-in-link');
    
  copyFolder(source, target);
}

function copyFolder(source, target) {
  var files = source.getFiles();

  while (files.hasNext()) {
    var file = files.next();
    var fileName = file.getName();
    if (!target.getFilesByName(fileName).hasNext()) {
      // Only copy if not exists
      try {
        file.makeCopy(fileName, target);
      } catch(e) {
        // Some files can't be copied, log them:
        Logger.log('Copy failed for ' + fileName);
      }
    }
  }

  var folders = source.getFolders();
  while (folders.hasNext()) {
    var subFolder = folders.next();
    var folderName = subFolder.getName();
    var existentFolder = target.getFoldersByName(folderName);
    
    // Only create folder if not exists
    var targetFolder = existentFolder.hasNext() ?
        existentFolder.next() : target.createFolder(folderName);
    
    copyFolder(subFolder, targetFolder);
  }
}