I don't think Backblaze's personal unlimited tier is going to easily support op's Synology. I'm sure there's a way to get it to work, but their B2 service integrates with Synology and is the appropriate route to take. Op's looking for redundancy. I wouldn't want to rely on an unsupported work around to guarantee my data when they offer a service that's targeted towards what they want to accomplish.

Backblaze B2 is probably going to be the cheapest and seamless options since they say it integrates with Synology NAS. It's $5/TB/month.

Microsoft Azure Archive storage looks like it might be cheaper per month, but that's just going based on storage costs alone. This guide has a pretty decent explanation of examples of the cost to upload and store it.

Backblaze B2 has the synology integration and op wouldn't have to think about the costs of being able to access or retrieve the data from something like Azure cold or archive storage tiers, since B2 is sold as a hot storage option.