Ways to control etag/Additional Checksums without configuring clients:

CopyObject writes a single part object and can read from a multipart object, as long as the parts total less than the 5 gibibyte limit for a single part.

For future writes, s3:ObjectCreated:CompleteMultipartUpload event can trigger CopyObject, else defrag to policy size parts. Boto copy() with multipart_chunksize configured is the most convenient implementation, other SDKs lack an equivalent.

For past writes, existing multipart objects can be selected from inventory filtering ETag column length greater than 32 characters. Dividing object size by part size might hint if part size is policy.