The closest equivalent I've found for light brown sugar / demerara is "cassonade." This is what it looks like. You can find it easily in every supermarket (Franprix, Champion etc) and it's not expensive. It's not quite as fine as I had been expecting and that I need to keep hunting for, but it's the same as a light brown sugar in every other aspect.
Indeed, it was very similar in texture. In the photo below, you can see what it looks like when it's just been poured into a frying pan with some butter:
It's a little bit darker than how I remember muscovado and has a slightly stronger smell of molasses, but in terms of taste once I'd made it into a sauce, it was almost exactly the same. Definitely a successful substitution.