haven't actually proven to be effective at stopping cheaters
This is what OP said, and it's completely correct. It's not that much impact in comparison to "regular" anti cheat systems. And both of those only detect either cheap/bad or known hacks.
Server-sided and data based anti cheats is what would actually be a huge step up. You're running a 8 K/D in a game where the best players are between 1-2? Banned. You just flicked two enemies within 100ms? Banned. Suspicious activity that's not that blatant needs to be reviewed.
The thing is - that's fucking expensive, complicated and needs to be done one a per-game basis, and since its just cheaper to throw you under the bus with a kernel anticheat and claim it's the best one, that's being done.
The smallest footprint for an actual scripting probably will be posix sh - since you already have it ready.
A slightly bigger footprint would be Python or Lua.
If you can drop your requirement for actual scripting and are willing to add a compile step, Go and it's ecosystem is pretty dang powerful and it's really easy to learn for small automation tasks.
Personally, with the requirement of not adding too much space for runtimes, I'd write it in go. You don't need a runtime, you can compile it to a really small zero dependency lib and you have clean and readable code that you can extend, test and maintain easily.