IInkscape is just there for us when we need it; it's one of those staple FOSS applications that just is. I've been using it since the Sodipodi days; that's more than fifteen years ago!

Inkscape is pretty extensible as well. It's not too hard to write plugins for it (in Python). I've did it myself, and love how a handful of people seem to find it of use¹.

1: https://inkscape.org/~jdhoek/%E2%98%85isometric-projection-c...