I already had a DSL and with a proper prompt and a checker tool, even tiny LLMs could build really good scripts in that language. Gemma 12B QAT was excellent.

Actually the tool-calling convention is a small DSL too.

I suspect the DSL ideally has to be similar to something in the LLM's training set though.

It doesn't have to feel too alien, otherwise the description of it has to be thorough and will eat up a lot of context.

Frontier models don't suffer as much of this limitation since they can grab onto a larger corpus of knowledge, but they're expensive.