Well the proposed solution is less simple than I thought: the distance ‘X’ is only 2mm. The cup spring thickness is 1.7mm, so it can only be compressed by another 0.3mm before being completely flat. It is not advised to push a cup spring until it is flat as the stresses in the spring become too high.
Adding a washer under the cup spring is therefore not advised, if the washer is about 2 mm thick (like the stainless steel one I found in a DIY shop), the cup spring will not be centered anymore and bolt 373 will not be screwed against axle 340.
Overtightening the bolt will damage the cup spring.
Adding a third washer between axle 340 and washer 364 is not ideal because it will not center perfectly and the inner edge of cup spring 367 will not be well guided.
On my bike, I replaced the soft steel washer 364 by a stainless steel one, which seems to be a much harder material. The original washer was quite deformed/worn out, and my first impression today is that the rear wheel swing movement has been reduced by almost 50% by bringing the preload of the cup spring almost back to its inital value.