@_elena I know it’s an floss project, but this is basically asking for free spec work, with the off chance of getting paid a little only if you win. Not many professionals will answer that call.
With their budget of 1500€, they can hire someone working fulltime for 2‑3 days maybe. The result will be much better, it’ll be an actual cooperation with proper briefing and feedback rounds, etc.
With a contest, they risk having most people just sending things they spent a couple of hours on, tops.