Yeah, I tried --gc:none. After some playing around, I ended up with:

  --gc:stack
  --header:"nim.h"
  --os:standalone
  --noMain
  --genDeps

In the end I got Nim code to run and was able to do a "hello world". As a hack I also used -d:android to trigger some conditionals that reduced the reliance on various dependencies. But it wasn't enough to be usable, and it was a bit unfortunate to have to do that.

Looking at those projects you linked though, clearly I didn't try hard enough! Great to see that people have had success.