Best Way to Backup a Large SharePoint Environment to External Storage

I recommend Veeam for backing up Office 365, especially for SharePoint sites and OneDrive. It’s pretty efficient because it allows you to get the documents directly into a file system without needing to restore them to SharePoint first. One thing to note is their licensing is a bit odd; it's per user, but you can back up an unlimited number of sites. You might need to consider the cost since Veeam usually requires a yearly subscription, which might not fit your tight deadline.

Using a Synology NAS would be a great option! It's known to work well with backups and doesn't cost anything extra. You might want to check if all Synology models work with the Active Backup feature, but it can be a cost-effective way to handle that data. Just remember you won't need a huge setup; a standalone NAS should do the trick.

Have you looked into Rclone? It's a free tool but does require some command line knowledge. You’ll need to get familiar with MS Graph to retrieve OneDrive and SharePoint IDs, but it’s quite powerful. A heads up though, Microsoft tends to throttle your speed, so you’ll experience fast and slow moments while transferring. If you want, I could share some of my scripts that make it easier!