S3 offers a flag that prevents modification or deletion for a set period, and afaik basically every S3-compatible provider offers that.
I use that along with a lifecycle rule to maintain my backup buckets on iDrive.
If you need a 'you cannot touch this and the provider has no way of allowing it' then you're talking specialized corporate talk-to-a-sales-person-for-a-quote, as you found out.
Edit: if you don't need cloud, there's options for WORM media from the humble BluRay to fancy SSDs that don't allow deletion.
Restic has been great for automating backups, and even letting me mount the encrypted storage to grab individual files. I like doing it this way since I don't have to trust Backblaze isn't reading my data - I know for sure that they can't.
Performance of storage that is both remote and encrypted is about what you would expect, but I don't need access to the data unless something bad happens.
I use backblaze storage with Kopia, which supports using object lock. Every time a backup is made the objects for it are locked for a configurable amount of time. I use 30 days, so an attacker would have to compromise my backup software for a month before being able to erase my backups.