I am writing from Storadera.
Would it be okay to add a new storage provider to your list?
Storadera is S3 compatible cloud storage and we have tested Duplicate in-house and it works well with our storage.
The service is in Beta at the moment but whoever wishes can already use it for free.
Our endpoint URL is eu-east-1.s3.storadera.com
We also provide our own manual for set-up with Storadera: