I build https://github.com/uroni/hs5 as replacement for single node use with a focus on high performance. I list other alternatives in the README there. Some short version:

Ceph: Robust, widely used for multi-node deployments. Would recommend for serious use.

RustFS: As an in-place replacement using the same storage format. Though, to me it is a bit suspect, e.g. if it uses fsync for durability.

seaweedfs: Multi-node alternative, that keeps the object mapping in memory (so more RAM usage and startup cost compared to alternatives).

Garage: Multi-node alternative, web-interface available separately. To me seems unsuitable for single-node use at this point.

VersityGW: Single-node alternative, which uses the same object=file in file system tree storage as MinIO (with the same disadvantages/advantages). It uses extended attributes for S3 metadata, so less filesystem overhead than MinIO/RustFS at least. Cannot find any Sync() or fsync calls in the code, though.