Starting reviews with way of Balance.

Overall, I am not feeling the theme of the competition running deep here. Having to wait till 17th level to get your quintessential ability means that for most of the game there is no sense of bunch of fives.

In terms of mechanics it also feels a bit all over the place, although this is probably inevitable. The fact you are trying to shoehorn abilities from 5 different elements into one class was always going to make it a challenge to have it feel like a coherent whole.

The individual abilities are also on the strong side. Healing for 3d8 for 1 ki? Being able to get mass curative effects before clerics bards and druids? This seems a bit much.

Likewise the fire effect for 12d6 area of effect is stepping pretty hard on the toes of fireball, especially as Ki comes back on a short rest. Even the fiend pact warlock with short rest fireballs as a major feature would see this as very strong (and a d10 fireball at level 11 would be 1/3 of the short rest resource of the class).

Sleep... an area of effect control element on top? Well I am not seeing the elemental connection to sleep but could be something here I am missing.

I guess I feel the class should be tuned downwards a little. It is really good at too many area with the subclass: damage, control, healing, shutting down casters... all on top of being a monk as well. Nothing wrong with flexability but I don't think you should excel in so many areas.

That said, some of the abilities themselves are cool and thematic. I think that things like your 5th element ability could be reworked to come earlier - this is both a cool ability, one that could compliment other monk abilities but also somewhat different from other classes so less likely to be stepping on their toes. Closest anti caster option might be monster hunter ranger (wis/dex/anti caster) so keeping a different feel to this might be useful.

I loved the imagery of the water container for the water ability. Building a material elemental components theme for the other abilities might be cool and could also be used as a point of balance?

I feel a bit harsh saying this - there is a lot to like and I feel like I have just focussed on the bits I don't which is unfair.