Turns out you can't control the blur, except for one thing : the blur is computed between each pair of consecutive frames, so if there are for example 480FPS, the blur will be very weak. So the "trick" is to render with a huge number (for example 480) of FPS and then bring it back to 60FPS (or 30 or whatever).
It's waaay longer to render but worth it

Thanks for the help

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