>>2188558I mean, your problem comes from trying to do your lines using strokes...
If you make your lines using fills, then you would have complete control over the thickness of the line on all the nodes.
Also worth noting that for best results, the nodes of a line should be directly across from one another and their handles as parallel as possible. With that, modifying thickness is just a matter of adjusting the separation between nodes and adjusting the handle length to maintain the correct curve.
Pic related shows a simple example of what I just said.
