I'm planning to do the same, but best practices on the 3-2-1 rule say that at least one of those backups should be off-site.
I made a backup of all of my stuff onto a 4TB external and left it at my parent's house last Christmas, but that's probably not what they meant by off-site. :)
That's definitely a legitimate offsite backup strategy. If your data changes more often than you visit, you may want something more interactive like a Raspberry Pi connected to that external drive that you can send files to.
If you use a Dropbox like solution, some of your data is already off-site in the form of the end user machines (e.g. laptops). It's not exactly a "backup" (in that it's not a timestamped snapshot), but it does mean your data is replicated.
But please be careful because if you use anything based on automatic synchronisation, such as RAID or a DropBox-like service, there is also a danger that anything you accidentally delete or corrupt will then propagate. If there’s no historical version saved as part of the system, or if there is but you don’t notice the problem before it disappears, you can end up damaging your “backup” copies of your data in exactly the same way as the original if something goes wrong.
I made a backup of all of my stuff onto a 4TB external and left it at my parent's house last Christmas, but that's probably not what they meant by off-site. :)