Rust

One thing that prevents me from letting users build their own templates is the scripting capabilities. A joker creating an endless loop could block the whole server.

I know little about servers, but can't you run the generation in a thread with a timeout or so? Or maybe compile to wasm and let the generation run on the users machines? I also know little about wasm, but I'd have guessed it has facilities to keep runtime and load under control.

What would be nice is a “safe” mode in which no access to the file system (include and sorts) and limited runtime makes it safe to let users build their own templates.

Ah, are you by chance shelling out to the typst binary? You can actually use typst as a library, and then you've got full control over the files involved, so it's "safe" in that respect.