Hadn't heard of SSDB before, looks promising, active, variety of users, broad client support.
Off the top of my head, if I were trying to front AWS S3 from DigitalOcean, I'd have gone with MinIO and their AWS S3 gateway[1]. It appears to be purpose built for this exact kind of problem.
You might face a language barrier when trying to find answers from the community: For example it looks like most of the discussion on the issues and pull requests are in Chinese. I do not believe Google Translate does well with technical terms.
(Of course this is a concern only if you don't read Chinese.)
Last I checked, DigitalOcean has a "Spaces" feature for file storage, which is compatible with the AWS S3 API (and they even suggest using the AWS client library).
I always found it odd that we can easily port apps, databases, etc., from one cloud to another, but for file storage/CDN it's always some proprietary solution like S3. AFAIK open source solutions never really took off.
Off the top of my head, if I were trying to front AWS S3 from DigitalOcean, I'd have gone with MinIO and their AWS S3 gateway[1]. It appears to be purpose built for this exact kind of problem.
[1] https://docs.min.io/docs/minio-gateway-for-s3.html