First of all, HBS does incremental backups on schedules. I do it every day both locally and to the cloud. I’m not sure why you think it only does full backups. That is incorrect.
You can definitely do what you want with Backblaze. I would check out QNAPs MyQNAPCloud service. The drive space there is very reasonable and you don’t pay for usage.
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Snapshots are really not meant to be your immutable backups. If you have data that you want to be sure is immutable, set up a WORM shared folder/volume and store your data there. You can snapshot that folder as well. If snapshots were immutable, it would eventually eat up all your disk space.