On the other side I realise I am doing things wrongly. If I have black bars it is because my screen is 16/10 while the standard is rather 16/9. It's up to me to adopt the most common ratio and base my design on it rather than tweaking Fusion to show things at screen that most users won't be able to see on their side.
I will just stick to the 16/9 ratio as the main conception base, my attempts to making a responsive interface causing more harm than good.
That's it. For a puzzle-casual game, ultra wide and unlikely 4/3 screens would have to rely on black bars anyway. And 16/10 users won't lose any vital information on their side.