Finally added some kinks to the gutter downpipe, which might qualify for a first-flush system:
The system will fill with the first 5 liters from the gutter before water starts going to the underground tank.
I still need add a small valve, or drill a tiny hole, to slowly let the dirty water out between rainy days.
The tee joint has a course leaf filter inside, I had to slightly adapt it to fit the rest of the plumbing