The Mystic Knight (Magical Melee DPS)

[Archwizard]

Hoo boy, okay. Couple things.

One, nothing in FFXIV has had an elemental weakness since 1.0 -- at least not outside of Eureka. All "elemental" damage typings are purely cosmetic, and are just treated by the game as "magic" damage. Without damage vulnerability effects (which were done away with in ShB) or buffs like Astral Fire or Embolden/Brotherhood, there are no functional differences between the damage types.
If the elemental wheel did still exist, every Disciple of Magic as they are now would be simultaneously screwed. And bear in mind that with the way Embolden/Brotherhood work as now, a purely Magical melee DPS would be unable to inherit physical damage buffs while facing the same cases of magic invulnerability as casters, making them fall behind other melee.
This is a large part of why, when discussing elemental types in the other Magic Melee DPS thread, we made a specific note that the En-element effects would have other benefits to the user, like AoE, resource recovery, DoTs, speed, direction negation, etc., and why I added the note to have strikes do partial physical damage with the magic scaling off of it.
Without those types of benefits adding complexity to the job, your design is very... simple.

Two, Holy and Dark don't exist in FFXIV as elemental typings, and spells that deal their "damage" (including WHM, BLM, PLD, DRK, SCH and RDM spells) are just classed as unaspected magic damage. Yes, RDM has separate White Magic and Black Magic finishers, but they have separate (albeit parallel) functional purposes within their own kit.

Three, "Darkside" is a classical DRK skill, and you're opening a whole can of worms with an Ultima-based skill before any existing caster.