EDIT: I believe I found the answer, it's something turned by this: https://ctatools.com/products/5065
Found some trash on the street that i'd like to take apart, but this screw is in my way.
It's like a hex bolt, but with 5 sides, and rounded (a bit like flower petals).
Here's another picture from the top:
I tried turning it with various tools (hard to get pliars in unfortunately), no success so far. The material is very soft, which doesn't help..
thanks, this might be just the comment I needed! Some of the screws are sunk into a very deep hole (~60mm), so it would be tough to cut a slot into those. But i'll give it a try on one of the more exposed screws, just to see if that gets me anywhere.
🎉