Love to hear you enjoyed it!
The menu transitions use this shader:
https://godotshaders.com/shader/universal-transition-shader/
I basically had a static background and had the Control nodes of the menu(Buttons, labels, etc.) be transitioned in and out with that shader.
So the old menu fades out at the same time as the new one comes in on top of it.
(each submenu has it's parent control and the children inherit the shader material)
I'm not that confident in visual stuff so nice that you found it cool 😎