Really great game and nice take on the theme. As someone who recently moved to Godot from other engines it was really helpful to see some of these behaviors in action.
When I reached the final boss I wanted more, the boss was a fun idea and I think there's still a lot of combinations of extensions you could expand on for new levels without needing to add more extensions.
I'm not sure what changing ExtensiON to ExtensiOFF does on the main menu or if it's just for show. I was also able to beat some levels in strange ways with the Drag and Drop extension as well as Screen Drag.