There you are. First, mythology cannot be the source of astrological symbolism. But assuming we accept that, look at the words you chose for Eris....
Neptune is said to be the Higher Octave of Venus, Harmony. Confusion, Misdirection, Deception, Frustration all belong to Neptune -- No Eris is necessary. Depression would be Saturn's realm, as are Order and Destruction. Eris is superfluous, and does not fit the desired parsimony.

We can remove Neptune from the pantheon, and assign those meanings to other planets as well.

In Greek mythology, Hades does not destroy or punish the dead; he simply holds them. I must therefore assume that the planet Pluto does not destroy. Clearly contrary to experience. To borrow meaning from mythology, rather than deriving it from celestial facts, is inconsistent. Astrology should, must be, a self-contained art without necessary reference to anything outside its own realm.

The original Neptune,who was of Cretan origin, had llittle or nothing to do with the sea. He was a god of horses and earthquakes.

New Age astrologers say that the three modern planets are social/collective and have little to do directly with the native of a chart. Read Robert Pelletier's descriptions of aspects of Pluto to see the outcome of this viewpoint. The first horoscope you meet with a prominent Pluto will show you his errors. There is no room for doubt that the three outer planets have very specific individual effects.

There is a big difference between astrologers who accept such pronouncements as valid on their face (they are the Book Astrologers), and the ones who constantly test and experiment.