hi guillaume
imo, the bezier tool in scribus is good for small tweaks, but i prefer doing more complex shapes in inkscape and then import or paste them into scribus.
in the specific case -- and if you really want to do it with scribus --, i would avoid "pulling the node" while creating the shape.
as soon as you have more than one point to "smooth", it gets hard to control it to get the result you want.
i would create an "edged" shape (just click on each node) and then double click on the shape to edit it nodes.
try to do it with as few nodes as possible.
do you need help for editing the handles on the nodes or that part is fine?