With all the discussion here about alternative programs to make vector graphics for mathematical figures, I'm a little surprised no one has mentioned Ipe (https://ipe.otfried.org/), given it's far and away the most popular tool in my field (and has good native support for LaTeX expressions).
It certainly doesn't have all the features that Inkscape supports (although it does have quite a lot of features), but the features it does have are implemented with a heavy emphasis on ease-of-use. It's sometimes easier to brainstorm within the program than on paper.
It's also plenty easy to add useful plugins in Lua: At one point I coded a plugin to check if a diagram of a graph contained a Hamiltonian cycle (by calling an external C++ program I wrote that used simple backtracking). I was able to very quickly experiment with which of my small graphs and modifications to said graphs ended up having Hamiltonian cycles, and was able to narrow down some useful features to think about.
It certainly doesn't have all the features that Inkscape supports (although it does have quite a lot of features), but the features it does have are implemented with a heavy emphasis on ease-of-use. It's sometimes easier to brainstorm within the program than on paper.
It's also plenty easy to add useful plugins in Lua: At one point I coded a plugin to check if a diagram of a graph contained a Hamiltonian cycle (by calling an external C++ program I wrote that used simple backtracking). I was able to very quickly experiment with which of my small graphs and modifications to said graphs ended up having Hamiltonian cycles, and was able to narrow down some useful features to think about.