addons-btree VS Gut

You mean this one right https://github.com/fian46/addons-btree ?

If you ever feel the AI logic getting away from you, you can try a behaviour tree or finite state machine.

And it's extensible by plugins! I like using GUT [0] for unit tests, and it injects its own editor tab with a custom UI... which is just another Control node [1].

[0] https://github.com/bitwes/Gut

I forgot to add that unit testing would be great for this. There's a couple frameworks out there, I'd suggest GUT: https://github.com/bitwes/Gut/tree/godot_4.

There are some downsides. GDScript is a dynamically typed language, although you can use static typing, which is slightly more error prone. It doesn't innately support things like unit tests, either, although there's a decent unit test plugin that is in the process of being ported to Godot 4. It's a bit slower than C# for non-engine functionality as well, although you'll probably only notice if you are doing a bunch of nested loop operations (in which case you should probably refactor and try to avoid that kind of thing).

Allows to simulate input https://github.com/bitwes/Gut/wiki/Input-Factory

I don't know if you're aware, but you can run some programs/scripts from the command line, even of tests. Using Gut, you can run tests from the terminal :

My solution in Godot 3 was to use sub-repositories. If I wanted GUT, for example, I'd fork the repo, move the contents of the addons folder to the root of the repository (and delete any extra root files), then import my GUT fork into my project as a subrepo. This meant that the entire module was stored as a single Git reference.

You can find the GUT library here: https://github.com/bitwes/Gut