github repo : https://github.com/gokudomatic/eco-state-machine
It's maybe yet another state machine for Godot, but it's pure gdscript and it supports imbricated states, events through signals, timers, and condition evaluation with method calls. It should be enough to do moderatly advanced systems. Also, it doesn't require to be extended and can be simply instanciated, which allows concurrent states.
It allows to make such kind of machine: