Selfhosted

I'm using Kopia with AWS S3 for about 400GB and it runs a bit less than $4/mo. If you set up a .storageconfig file it will allow you to set a storage level based on the file names. Kopia conveniently makes the less frequently accessed files begin with "p" so you can set them to the "infrequently accessed" level while files that are accessed more often stay in standard storage:

{
  "blobOptions": [
    {
      "prefix": "p",
      "storageClass": "STANDARD_IA"
    },
    {
      "storageClass": "STANDARD"
    }
  ]
}

Options if it's to protect against local disasters such as fire:

1. Having a NAS at a family member / friends house as a backup location for your NAS (over vpn) is an option. Works best if they also need an offsite backup with you being able to spare space for it on your NAS in return.
2. Having at least two usb drives as backup locations for the NAS and rotated as often as you think necessary and having at least one stored offsite at a family member / friends house.
3. Rent a proper 1U rack space in the city data centre and setup your own "cloud", definitely the most expensive option and total overkill if offsite backup is the only reason.

Personally I would probably go for option two and bring the usb drive with me for a weekly coffee with my parents, they'd enjoy the visit and I enjoy knowing that my backup isn't in the hands of Amazon. I'd go for option 1 if my internet was better.

I am looking for a solution for a ~1TB collection, and the Glacier Deep Archive storage tier is barely above 1$/m for the lot. You may want to look into it ! If I remember correctly, the retrieval (if you one day need to get your data back) was around 20$ to get the data in a few hours, or 2$ to get it in a couple days.

B2

Seems like a good way to do it.

Keep in mind Kopia has some weirdness when it comes to transferring repos between filesystem and S3, so you'd probably want to only keep one repo.
https://kopia.discourse.group/t/exported-s3-storage-backup/3560

Backblaze B2 is a cheap S3 provider. Hetzner storage box is even cheaper, but it doesn't support S3 natively, so you're likely to run into issues with the kopia repo compatibility I mentioned.