Canopy usage - Is the idea feasible in this way?

A greened garage roof is designed for these loads, you might also be able to put a few pebbles on it.

Best regards
Sabine

You plan the drainage for these flat roofs as needed [emoji2]

Wherever I put a flowerpot, there always remains a wet or damp spot under the pot and at the edge in humid weather. Eventually, sand settles there, and it gets dirty.

The rest would be a matter of taste for me. It may look different and funny to have a few metal animals or colorful plastic windmills on sticks on the roof – but it would be worth considering whether it’s not silly.
But there are those who can’t get enough of such decoration stuff, so much that they even use a free roof for it [emoji6]

From where do you constantly see this roof?

You can basically look at it from the street, of course only if you lift your gaze a bit. In our house, the direct view would be from a future child's room (from a child that doesn't exist yet...). It may be that neighbors constantly look in certain rooms, but we couldn't care less about that... We're more the "playful - different - silly" type, yeah, that might be true :-D But of course, you don't have to overdo it, especially if you make the effort without it being worth it afterwards...

So on the topic of "we just dump earth there and sow plants" without it being planned beforehand, I would be cautious! In such a case, other (root-resistant) roofing felt would also be planned. Normal roofing felt would be damaged by roots over time.