" but if you press space bar early, it skips the writing step": I found out, that in my project the input map (Project > Project Settings -> Input Map >") ui_accept also used the space bar, therefore it skipped the writing step! Just remove the spacebar from ui_accept solves the issue!
Viewing post in Godot 3 Dialogue System comments
Finishing the line and continue to the next line are mapped to the same button based on how this code is written; remapping the spacebar to the "e" key for instance will still cause the line to skip or finish when you press it. No matter what you remap the button to, you can still mash the one button in order to finish the dialogue. It needs to be rewritten to provide the functionality I'm talking about. I wanted to remap the skip text feature to a separate button in order to prevent players from accidentally missing dialogue. Keeping skip on a separate button makes the action more intentional.