I wish there was someway to do this without, for instance, a state machine or cases within the _process function; instead, _process fuctions that have an integer argument like set_process(true,0) ---> _process(delta,0)
@1000h Can you elaborate on why you feel this would be beneficial? I don't feel that implementing conditions in _process is a huge problem, though I can see how multiple nameless _process functions can harm readability.
To process a state machine, you only run one thread that check currently possible transitions.
If you really need threads, you have http://docs.godotengine.org/en/stable/classes/class_thread.html