YAML might be terrible for the reasons given elsewhere but I really want something YAML-like (maybe TOML, maybe something else)

For me, readability is king both for myself and because I sometimes want non-developers to be able to hand-edit files.

Any curly brace format is a non-starter. Too much clutter, too easy to create invalid files that aren't obviously invalid at a glance.

Significant white space does have the advantage of meaning exactly what it appears to mean and being intuitively understandable to most people.