I back up to the cloud. Do I also need an external drive back up?

See the article Other ways to back up and restore files - Apple Support

Use cloud-based services

Cloud-based services don't require you to connect an external hard drive to your Mac. For example, iCloudcan automatically store your photos, calendars, contacts, notes, passwords, and more. And it includes iCloud Drive, which can store documents, desktop files, and other files of your choosing. To access those files, you just need to be signed in to iCloud. Other cloud-based services are available from third parties.