`rclone serve s3` is actually a thin wrapper around this: https://github.com/rclone/gofakes3

That repo is a fork of this project: https://github.com/johannesboyne/gofakes3

They bill it as being for testing, but it works great if all you want is a no-fuss S3-compatible API on top of a filesystem. I've run it on my NAS for a few years now to provide a much faster transfer protocol compared to SMB.