For your photos, you can use iCloud as a backup by adding the photos from your external disk to your iCloud Photo Library, either through the Photos app on your Mac or via iCloud.com. You're probably wanting to keep these photos separate from your main library though? The problem is you can only have one single iCloud Photo Library. As a solution, I would suggest creating a new album in your iCloud Photo Library which contains all the photos from your external disk, as a way to keep things organised.
(A more complicated solution, if you have a Mac, would be to create a new Apple ID through Family Sharing. Then, create a new user account on your Mac, and log in to iCloud in the new account with the new Apple ID. Now you have a whole separate iCloud Photo Library but which shares your 2TB storage. You can upload the photos from the external disk and just leave it there as a backup. Personally I think the simpler option will do.)
iTunes is more complicated.
You can't automatically back up iTunes to iCloud in the same way you can back up your photos. Instead, Apple offers a service called iCloud Music Library which is available to all Apple Music subscribers ($9.99/month, or $14.99/month for a whole family) and iTunes Match ($24.99/year) subscribers.
iCloud Music Library scans all your songs in iTunes and tries to find matches in the iTunes Store. If it succeeds, the full quality iTunes Store version is made available to you (and anyone else using Family Sharing) for streaming and downloading on any device at no extra charge. If it fails, the original song from your iTunes library is uploaded and is also made available to you and your family for streaming and downloading anywhere.
So while this isn't a backup in the traditional sense, it is a safety net. If your external disk was destroyed, all you would do is re-download all the songs from your iCloud Music Library.
(Of course, you could just copy the iTunes files from your external disk into your iCloud Drive as a rough-and-ready backup solution, but it wouldn't update over time, so you would have to make regular, manual backups. And since you can't easily track the changes that might mean uploading huge amounts of data each time.)