If online doesn’t work, maybe a carry-it-yourself rotation of 2 or 3 drives to friend or future job would do? This can hurt the “off-site” goal unless 2/3 off-site-at-a-time is enough or you buy lots of drives to rotate.
This would probably be best done by dividing your 10TB into smaller chunks, one per destination, which would be good anyway because a single 10TB backup with Duplicati might call for special care such as Choosing sizes in Duplicati so that performance (including that of recreate) doesn’t die from the scaling. There’s also a question of how much of the 10TB backup you want to lose at once if the backup breaks.
Some people also try to divide backup source based on use, to try to get online backup of changed files, while letting old files get a different backup. I don’t know if this fits your usage pattern, and it can be tricky.
Filter: older than x days would help with this plan, but such a filter isn’t built-in yet, so you use scripting… Possibly this would work poorly with VM images anyway (scattered changes). Can you backup in guest?
In the realm of emerging technologies that try to undercut traditional cloud storage from companies that purchase their own drives, there’s decentralized storage such as Sia, Storj, and others that rent storage from those who have extra, and resell it (in a more presentable more-available form) to those needing it. This might bring price down from $5-$6/TB/month for inexpensive traditional storage such as Backblaze B2 or Wasabi, to $2/TB/month, however cryptocurrency (and possible speculation) is typically involved. That $2 price is from their own web site, however I found other numbers on other sites that track pricing.
Sia Decentralized Cloud is the Duplicati setting, however forum reliability reports haven’t been that good, possibly due to the sometimes-there-sometimes-not nature of their backends in spite of the redundancy.
Very few companies want to lose money by providing storage below their costs. One that’s still willing is Backblaze with their own client. The catch is it’s aimed at flat-rate backup for users who favor simplicity. Large users (like you) who refer less technical friends (like yours) with less data average out acceptably. Client has some other catches that technical users might not like, such as not supporting server OSs…