Flat roof waterproofing terrace / flooring

Hello dear forum,
I have been reading for a while and today decided to register here because I really don't know what to do anymore.
We are currently building new and are dealing with the terrace, we want to realize it on our garage, which shall have a flat roof. The internal build-up height is 14cm, the balcony door is threshold-free, so the external build-up also remains 14cm.
Currently, the floor slab does not have a slope. Planned / was actually a sloped screed, which is supposed to stretch over 7m.
The architect advises a 0-degree roof, which we rather have doubts about, so now the sloped screed instead.
A 40mm XPS insulation is to be used and a foil with fleece is to be applied on top, so the build-up is 43mm, which is already given.
Everything else is open. Are there any ideas on this?
We are quite flexible regarding the covering.

Slope insulation and then elevate the terrace. It is best to collect the water in a cistern. Actually, such details are planned before starting. But it is more of a "simple exercise".

Slope screed with insulation or slope insulation, both possible and common.

But I haven't quite understood your heights yet?
If the door is to be threshold-free and a slope of 2° over 7m is necessary, I assume away from the door, then you already have a 14cm height difference just from the slope. There is then no space left for flooring. Therefore probably the hint about the 0° roof, which is undoubtedly technically feasible without problems, but will cause additional costs.

Thank you for the answers. The thing with the slope insulation 2-degree inclination is unfortunately rather off. Do you have an idea for a covering? We have already thought about a 1-degree inclination in the screed so that we would at least have some slope and then put a relatively thin structure on it. The covering would then still have 7cm - 4.3cm -> 2.7cm left. The [Zisterne] is a good idea, thanks!

Propping up. The water runs through the covering and the slope underneath is balanced so that the covering is even.

Is a 1% slope sufficient and what type of surface would you recommend for a thickness of 2.7cm?