With every alternative, the prevailing issue is the fact that your data is as safe as the company your data is with. But I think this can be remedied by doubly external backups.
Backblaze is like if Amazon spun AWS S3 out as its own business (and it added some backup helper tooling as a result) though, I wouldn't really worry any more about it. You could write a second copy to S3 Glacier Deep Archive (using B2 for instant access when you wanted to restore or on a new device) and still be much cheaper.