AWS, GCP, and Azure all offer cold storage for about $1 per TB per month. If you want any cheaper you need to build a second NAS.
You could also take the awkward route and add one or two large drives to your desktop, mirror there, and back that up to backblaze (not B2).
The other suggestions you got for hot storage strike me as the wrong way to handle this, if you're considering $80 per year per TB for backups then just make another NAS.
For the OP - be careful with AWS, the closest pricing to one dollar per terabyte is S3 Glacier Deep Archive and you'd be surprised how expensive a full restore can be in the event that you need to do so in terms of restore pricing, egress cost, etc.
Another NAS isn't really a good solution (unless you can place it in a different house) - the goal of a cloud back up is that it's offsite.
You could also take the awkward route and add one or two large drives to your desktop, mirror there, and back that up to backblaze (not B2).
The other suggestions you got for hot storage strike me as the wrong way to handle this, if you're considering $80 per year per TB for backups then just make another NAS.