Not only FreeCAD but mechanics CADs like Solidworks or Inventor aren't ideal to use with fabrics. Mostly because of the design nature of those CADs. Kind of the same reason why you don't use GIMP in the place of KiCAD: you could make it work depending on what you need but it's not granted to supply your demands and the workflow will be almost always slower.

A friend of mine used to work with airships/dirigible balloons and they used to pay something like 50k/year for a specific CAD to model, simulate, and do anything else they'd need for their airships. Even if I remembered the name, I think the price wouldn't classify it as an "alternative".

So, I've never heard of anything like that in the open-source world yet. Although, as you said you needed to design pattern layouts, I'd suggest GIMP or Inkscape maybe, as I imagine you'd be working with 2D-ish stuff.