I ended up getting a bit further with this by upgrading to Emacs v29 (which natively supports tree-sitter), and using the elixir-ts-mode package.

Doom’s docs do specifically call out that Emacs 29 is unsupported - but I haven’t found any issues after ~1 week of use.

Here’re the rough steps I took:

1. Upgraded to Emacs 29 (I’m using emacs-plus on macOS), so for me this looked like:

   # I'm using emacs-plus on macOS
   brew uninstall emacs-plus && brew install emacs-plus@29
   

2. Installed tree-sitter:

   brew install tree-sitter
   

3. Added the elixir-ts-mode package to Doom config:

   ; packages.el
   (package! elixir-ts-mode)
   

4. Configured .heex files to use elixir-ts-mode instead of web-mode:

   ; config.el
   ; Elixir: Use elixir-ts-mode instead of web-mode for HEEx
   (use-package! elixir-ts-mode
     :mode "\\.heex\\'")
   

5. Followed instructions on heex-ts-mode README to install grammar, YMMV, but for me it was as easy as M-x heex-ts-install-grammar.

6. $ doom sync and restarted Emacs.

7. Verified it all works by opening a HEEx file and running M-x treesit-explore-mode. You should see it print a HEEx AST, like this:
   

   

   CleanShot 2023-04-24 at 10.39.54@2x2378×1572 284 KB

You could also use elixir-ts-mode in .ex files to get syntax highlighting in ~H sigils, and then disable Doom’s default elixir config altogether… but I’m leaving that experiment for another day!