How immutable do you need?

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.

BorgBase allows for append-only backups.

I use Backblaze B2, but stored in an encrypted Restic container, set up using this guide:

https://helgeklein.com/blog/restic-encrypted-offsite-backup-with-ransomware-protection-for-your-homeserver/

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.

Backblaze B2 has the option: https://www.backblaze.com/docs/cloud-storage-object-lock

Cheap storage too.

How much data are we talking about?

A free mega.nz account should be fine for everything except family fotos and legally obtained music/movies.

Restic or Borg on your side, a safe and remote destination on the other side.

use restic, with backrest web GUI, and cannot be happier.

As for remote site, I use a remote machine I rent, but there are plenty of providers around, shop a bit... Or find a friend for reciprocal backup?