I just discovered the amazing stretch ratio option for containers in Godot!
Stretch Ratio: The ratio of how much expanded controls take up the available space in relation to each other. A control with "2", will take up twice as much available space as one with "1".
I always used transparent separators to get this effect. This is a much better solution!
https://docs.godotengine.org/en/4.4/tutorials/ui/gui_containers.html#sizing-options