One other option is using Representations, especially where you want to do a cartographic distortion of the boundaries without actually distorting the underlying geometry. Representations gives you more control over such things. The interleaving of hatched boundary lines is also possible using Representations. A boundary shared by all six years would probably be best represented by the interleaving of the earliest and latest dated boundary colors in a well aligned hatching design. Representations may let you achieve that.
How far you take this and what approach you should favor depends on the amount of importance you place on this one map and the number of other areas this type of cartography needs to be applied to. A high volume set of maps may need to rely on more generic steps that can be largely automated or scripted over approaches that involve a lot of custom work, while a small set of very important maps may warrant the time and expense of a highly customized workflow.