My https://github.com/uroni/hs5 is designed for this use case.

One notable thing is that compared to MinIO (and others) it does not store the objects as individual files. I also have DuckDB directly integrated.

The readme has a comparison to Garage, seaweedfs, RustFS and Ceph.

Very cool, I like the conceptual simplicity of it. Even I can understand it, at least at a surface level :-)

Is there a safe way to make backups of the index.lmdb and data0 files without stopping the server?

Not without snapshot currently. I guess I could collect all writes in the (optional) WAL file while a backup is running, then the files would be consistent.