There are also C++ and C backends at
If having a JS compile option is not a requirement, there are 2
more Ocaml/ReasonML-like BEAM laguages, https://github.com/alpaca-lang and https://github.com/lpil/gleam
(I'm kinda partial to Gleam, it seems really clean)
Also, pine was a good name! but I respect the practicality of the rename.
A "bespoke backend, going to assembly directly" does not preclude the option of integrating with an Erlang/Elixir project. In fact, I think that you could do so via ports or NIFs in such a way that the caller wouldn't know the difference. I think of the Erlang JSON libarary, jiffy. It's largely implemented in C for performance, but the language it's written in doesn't seem to matter in practice. As long as there is an easy way to integrate it into your Erlang/Elixir project.
I think an "Elm on the server" solution would work much the same way. Focused on making a clean integration point—then choose whatever implementation leads to the most performant and reliable artifact.