I, too, feel as though the whole subclass should simply be completely reworked. In emulating ATLA with the 5e Monk chassis and gameplay restrictions, they ended up with what just became a mess of disparate elements (heh, get it?). I think the addition of spellcasting is mainly uninspired and dull, while also not lining up with the fantasy of being a 'bender' all too well. Some of the more uniquely designed disciplines do fit with the 'bender' theme, but stuff like 'cone of cold' and 'gaseous form' is just kind of "eh". Where's the stone levitation? Riding on a ball of wind? Blood bending? Making shelters and houses from earth? The fantasy isn't fulfilled, nor can it really be fulfilled if we're just going off of ki and short rest resources (note that much of the bending in ATLA was basically at-will power, which doesn't fit with 5e balance).
Course, if I were to redesign it, I would depart from the ATLA four elements style entirely, and expand it to 5 elements, based on the ideas of Chinese Wuxing (with wood, fire, earth, metal, and water), since I think that fits the monk's archetype much better. Giving at-will abilities that tie into these elements would be paramount, if not cantrips or cantrip-esque features (I think Four Element's 'Elemental Attunement' discipline actually did this quite well), then at the very least the ability to change your martial arts damage to a specific elemental damage type. In addition, more unique abilities that aren't just "you can cast X spell using Y ki at 17th level", so the feeling of being a cool monk warrior controlling the elements in meaningful ways isn't gated by absurd level requirements. Perhaps having many abilites which each upgrade in power as you gain levels might be a cool option. Regardless, it isn't good the way it is, and I don't think it can be good without huge changes to how the subclass works.