Your default levels for ZSTD seem a little wonky:

• TURBO: ZSTD is missing, it should be -5 | --fast=5 (the fastest level currently available). This should push it into LZ4 territory.
• EFFICIENT should probably include 3, ZSTD's default level.
• OPTIMAL (meaning best trade off?) should probably be 20, as 22 balloons memory usa

It is gaining popularity and it seems that OCaml bindings are abandoned: https://github.com/ygrek/ocaml-zstd

See format description https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md

And decoding testing tool https://github.com/facebook/zstd/tree/v1.3.4/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing