Inverted roof and drainage - renovation question

Hello. I have had a balcony newly sealed with [Schweißbahnen]. Now I would like to securely drain the water and have a problem understanding. Under the balcony there is a room, so I want to insulate it with XPS boards. Google shows that with a so-called inverted roof, these XPS boards are laid directly on the [Schweißbahnen] and then gravel or crushed stone is applied on top. However, for some reason I would like to have gravel under the XPS boards to create drainage. But yesterday, I was told at the hardware store that the XPS boards would not like that. My desired setup specifically from bottom to top:

- [Schweißbahn]
- Separation fleece
- Gravel, crushed stone, granulate... some kind of drainage
- XPS boards
- Chippings
- Some kind of floor coverings that fit together like a puzzle.

I am interested in your opinion about my desire for drainage beneath the XPS boards. Unnecessary? Sensible? I am concerned if the XPS boards lie in water and prevent it from draining smoothly.

Sandwich made of gravel? I would say then the insulation effect of the XPS boards is practically zero. How are the boards supposed to insulate the lower room if you create a cold bridge made of gravel yourself?

If it becomes leaky, it is simply leaky and has to be redone.

Nothing will become leaky there for now. I am merely wondering how the water is supposed to effectively reach the drainage when the XPS boards lie on the waterproofing membranes and then on top of that the gravel or whatever else.

A slope of 1.5 - 2.5% is required