You call out 2d and mention mills and routers, so after reading the page I'm thinking I wouldn't be able to design a piece to be put in a CNC lathe ala the hub in the lower right of https://canev.com/cdn/shop/products/CEV5205_1024x1024@2x.png..., which has multiple holes (sometimes threaded), including threaded setscrew holes tangential to the primary axis.

Not as-of-yet --- it's 3-axis at the moment --- hoping to eventually add support for additional axis/axes.

Have you looked at grblgru.com ? Not opensource, but does have CNC lathe support.

It's not a serious problem I have, just saying I like OpenSCAD, I have been able to generate every shape I've ever needed to create, and it would do everything I want if it was possible to generate true curves (because I farm out the manufacturing).

Do you have a sample DXF (which uses arcs) which describes a lathe project?

I dug up an OpenSCAD from some backups if that's interesting to you https://pastebin.com/xwhrLwQH

I do not have any DXF of something like this.