Filestash (https://github.com/mickael-kerjean/filestash) has a s3 gateway plugin that I made. It proxy the S3 traffic to any downstream storage: SFTP, FTP, another S3, SMB, NFS, IPFS, ...

Almost went past looking at this option, it has a far bigger ecosystem of both storage and auth plugins and otherwise looks to be more mature than I might have guessed from the passing comment.