Always always always have 3 places where you store your data. Your main data store. Another storage location and then one offsite. And like others have said...check your backups occasionally to make sure you can successfully restore from them.
It's not a backup until you've proved you can restore!
The trouble was that I could restore; except the only copy of the backup was on the disk itself, and hadn't actually been uploaded to Filen.
Sorry about your data loss.
I self host an instance of nextcloud on a headless machine in my home. It gives the benefits of typical cloud based services, but you control the storage. And if you sync with at least one other machine you have a full backup besides.
Of course no solution is perfect. This requires extra time, effort, and hardware. And having my data in one physical location has it's own risks. But I still prefer it.
So, something to consider if you or whoever is reading this isn't aware of that option.