Oh drat, they used to have free examples you could play with. (Edit: they still have a free plan, after signup, so I still intend to try it)

I've seen Loopy before, but Sheetless looked more like existing tools with storied histories (Stella etc).

Like any forehead-smacking moron who can code I have sometimes considered writing my own because the thing I miss most of all in systems dynamics tools is being able to use git and write tests. But I know now that writing real software is really hard.