I was satisfied with Tuareg + Merlin for OCaml development in Emacs, it just worked for me and didn't break when I upgraded packages, but yes, this being from bbatsov is a strong incentive to try it out. My only concern is that it uses tree-sitter, which I try to avoid because of the messiness of the JavaScript ecosystem.

I think tree-sitter's relationship with JavaScript is entirely syntactic. You don't need any JS runtime installed to write grammars, because technically tree-sitter CLI already has a JS runtime included and using that it converts your grammar first to an intermediate JSON format, then it generates parser code in C. And then this C code gets compiled into a shared library, which is what editors like Emacs use, so to use tree-sitter modules you definitely don't need a JS runtime either.