In that case, Kakoune[1] (Helix’s main inspiration) is probably more your jam. You get an RPC interface and are free to script it from anything you want (shell to Rust is about the range I’ve encountered). It does mean that you don’t get the batteries you get with Helix (e.g. LSP support) and need to bring your own (e.g. kakoune-lsp[2]).

[1] https://kakoune.org/

[2] https://github.com/kakoune-lsp/kakoune-lsp

Is including batteries the main reason helix seems to have started taking off, while kakoune hasn't?

I use kakoune, because I like the client/server architecture for managing multiple windows, which helix can't do. The less configuring I do the better, but I've hardly done any in the past year. It's nice to have the option.

I do use kakoune-lsp and kak-tree-sitter.