I had mid-job stalling issues (after 5 hours or so) and I didn't trust the electrical isolation of USB but I also wanted more I/Os to add a laser. The stall issue ended up being fixed when the ethernet module was in shipping by grounding the machine frame. But that little ethernet unit has been mint, a lot smoother than the YongNuo USB breakout that came with my machine. It doesn't work on its own, it needs a breakout board - but the $5 ones on eBay work fine.

Thing with linear rails is that they're supported the whole length, as opposed to curtain rods which are only supported at the end. The carriages are also a lot stiffer in rotation than the curtain rod sliders.

Never had a problem with over heating, that said I've never run the spindle at anything near full load. I've stalled it many times with over aggressive feed rates but that's with bigger bits and lower RPM.

No Mach3 required for the orange box or GRBL. In your workflow you typically have CAD, CAM, Post-processing and machine control.

CAD - draw the things. Inkscape for 2d, FreeCAD, Fusion, Solidworks etc etc etc for 3D
CAM - work out the tool paths. There's a plugin for Inkscape. Fusion, SolidCAM, HSMWorks, etc etc etc.
Post - convert the CAM to machine specific instructions eg G-code. Usually bundled with the CAM in some crappy esoteric programming language so you grab the closest post out of their library and tailor it to your machine.

Most CAD/CAM packages have all these three bundled together, but not all.

Last is the motion control. Mach 3 / LinuxCNC / UCCNC etc on a PC. The orange box. GRBL on an Arduino with appropriate shields. The big machine panels from Siemens, Fanuc, Heidenham etc.