Xeviat
Hero
I like the idea of a bonus action for the spells but I'm also a little uneasy about it. Do we really need bonus action Fireballs?
I would reduce the cost of the spells first and grant some cantrips before making everything a bonus action.
So, Fireball is a little high damage, but lets see what this would look like.
For 1 action, 1 bonus action, and 4 ki, a 9th level monk would have 2 attacks (generally melee or very short range thrown) for 1d6+5 damage and a fireball for 8d6 to everything in a burst (including self potentially; 28 damage); for a potential single target total of 45. 3 extra rounds of no ki usage would be 3 attacks at 1d6+5, 25.5 each round for 76.5 over 3, plus the 45 for a total of 121.5 to 1 target max and 28 to multiples.
Over 4 rounds, for 1 action and 1 bonus action and 1 ki each round, the same monk could do 4 attacks each round, or 34 each round, or 136 max single target.
The burst potential is definitely higher, but I think this highlights an issue that the monk has no burst damage potential. A paladin can smite with higher level spells but a monk is always stuck using 1 ki for flurry of blows. Flurry over 4 rounds is better than 1 fireball and martial arts for 3 rounds. And that's a high damage spell for it's level.
Remember, this is the Elemental Monk's class ability, extra stuff to spend their Ki on, yet they get no extra ki to use them. Many of the other monk subclass features don't cost ki.
I would just be worried about any buffs to the Elemental Monk requiring subsequent buffs to the Sun Soul Monk.