Versity Gateway looks like a reasonable option here. I haven't personally used it, but I know some folks who say it performs pretty great as a "ZFS-backed S3" alternative.

https://github.com/versity/versitygw

Unlike other options like Garage or Minio, it doesn't have any clustering, replication, erasure coding, ...

Your S3 objects are just files on disk, and Versity exposes it. I gather it exists to provide an S3 interface on top of their other project (ScoutFS), but it seems like it should work on any old filesystem.