slvs_py VS FreeCAD_assembly3

If the general python API isn't enough the assembly3 docs have an interesting example which might give you some ideas for moving objects along within their constraints.

I think Realthunder's branch may have midpoint constrain you're looking for (https://github.com/realthunder/FreeCAD_assembly3/issues/51).

Also if you use this fork of freecad: https://github.com/realthunder/FreeCAD_assembly3/releases its much better at letting you attach sketches to faces without it borking things.

Take a look: https://github.com/realthunder/FreeCAD_assembly3/wiki/Topological-Naming-Algorithm

FreeCAD is still in development and you may have a mismatch of versions. Try .17, .18, .19, and .20 dev versions and try to find the one that doesn't show empty sketches. It may have even been made with the RealThunder branch.

Fusion 360 is probably the best choice to start. It's free and easy to use. But there are quiet heavy limitations as autodesk changed their licenses. I recently switchend from fusion to freeCAD and I'm pretty happy with it. I highly recommend Linkstage3, a freeCAD fork by a guy called realthunder. https://github.com/realthunder/FreeCAD_assembly3/releases

If you haven't already, take a look at Realthunders Linkstage branch. That's the one I usually use, although it can lead to adapting design practices that are not ideal.

FreeCAD fork linkstage3 supports expressions for patterns which I imagine would allow you to do this.