You can create your own Autoload Nodes (singleton system in Godot) to save/load/access game states and game progression.
There are 2 powerful dialogue addons for Godot: dialogic and godot_dialogue_manager.
godot_dialogue_manager is a lightweight Godot dialogue solution. It provides many dialogue utilities through code. You can easily make your own balloons and control the dialogue via code.
Dialogic is a general-purpose Godot visual novel solution. It includes many features (characters, portraits, etc) to create a VN game with minimal coding, similar to Ren’py and RPGMaker. However, the dialogic 1 is for Godot 3 and dialogic 2 is still in-development and marked as Alpha. But it’s already powerful enough to use.
In addition, both of them use several Autoload Nodes to control the dialogue process.