Pilou have you tried the Power Stroke in Inkscape ? As with Spiro it makes it
very easy to achieve long smooth changes in a curve with just a couple of points
and a great option of closing the ends with rounded caps.

Fat line is great but you cant achive changes in width as easily and smoothly as
with Power Stroke. But I am ging through all possibilities in MOI as I would
prefer to stay within MOI than to move between INK and MOI.

Big thanks to your contributiona by the way !