I keep trying mind maps and getting frustrated because my mind works differently. Anybody know of ones that let you start out non-hierarchical?

For me, key nodes are often obvious long before relationships are. E.g., if I'm working with post-it notes, I might write a bunch of notes, cluster them, winnow, and only then want to start locking in relationships between items. Has anybody seen something that's pretty straightforward that supports that approach?

I'm the same as you. Obsidian[0] is non-linear and has worked for me.

0. https://obsidian.md/

Looks great is this Free Software? I want to hack on this thing, where is the code?

If you want opensource and local storage look up Dendron. It is a VSCode extension in a similar vein, uses markdown so easily track changes in git/etc as well.