Bumping this topic because I just opened a pull request on GitHub to add ReScript support for ALE. Any Vim+ALE users please feel free to review it or copy the code to your configurations until it’s merged.
I recently returned to a ReScript project after a haitus, and lack of auto-formatting and LSP support was something I urgently needed to fix. A lot of the advice on these old threads has gone stale due to changes in the ReScript toolchain, so hopefully this PR code will be more future-proof.