Here’s the Elixir-related stuff from my ~/.config/nvim/coc-settings.json file:

{
  "coc.preferences.formatOnSave": true,
  "coc.preferences.formatOnSaveFiletypes": [
    "elixir",
    "eelixir",
    "heex",
    "json",
    "jsonc"
  ],
  "diagnostic.checkCurrentLine": true,
  "diagnostic-languageserver.filetypes": {
    "elixir": ["mix_credo", "mix_credo_compile"],
    "eelixir": ["mix_credo", "mix_credo_compile"],
    "heex": ["mix_credo", "mix_credo_compile"]
  },
  "elixir.pathToElixirLS": "~/.local/lib/elixir-ls/language_server.sh",
  "elixirLS.dialyzerEnabled": true,
  "tailwindCSS.includeLanguages": {
    "eelixir": "html",
    "elixir": "html",
    "heex": "html"
  }
}

I’ll be honest, I wouldn’t be surprised if some of those lines aren’t actually doing anything, and might just be residue left over from a previous time where I was in the same position as you.

The Tailwind stuff is for some CoC Tailwind plugin, and the pathToElixirLS is where I saved a copy of ElixirLS that I downloaded from their GitHub releases page.

As for the Credo stuff, well… I have no idea when or why I put that there. Might need to do some spring cleaning for my cluttered CoC config…