It's a good thing players of certain classes don't have to smash their keys any more.
These are classes like Aran (Triple Swing and Beyonder), Shade (Punch skills, e.g. Bomb Punch) and Battle Mage (Blow skills, e.g. Finishing Blow).
However, there are still many classes that are painful to play due to the constant need to tap keys, and I suggest making these either keydown, like Aran, or one tap activates all hits, like Shade and Battle Mage.
They are as follows:

Demon Slayer's Demon Lash and morphs, not sure if Demon Awakening has this, as I don't have a 5th job Demon Slayer. I suggest that it's changed to one tap activates all attacks, for the sake of jump-casting.

Angelic Buster's Trinity, I suggest keydown, as the following hits require the connection of the preceding hit. Or since it can be jump-casted, a single-tap system where the following attacks are only activated if the preceding attacks hit (though this sounds complicated).

Illium's Radiant Javelin and Radiant Orb, while these are currently able to be linked with the in-game macro, the attack speed suffers greatly, and the only optimal way is to tap them one after the other, repeatedly. This is only made worse by the Javelin being the main grinding skill, but the Orb is needed to keep up Bless Marks, which function like Luminous' Black Blessing. I would suggest keydown, where if both skill keys are held down, Illium would attack like the keys would be tapped repeatedly. The alternate is speeding up the in-game macro to the character's current attack-speed.

Blaster's Revolving Cannon, I would suggest it either changed to auto-use like Ark's Spell Bullets, where the setting is saved even when logging out, or like Night Walker's Shadow Bats, where it's necessary to turn it on every time you log in, but can be turned off without going to Skill Tab.

This issue is really close to my heart as I would probably main either Illium or Angelic Buster, but I had to give up playing them because my fingers couldn't keep up and I refuse to use illegal (out of game) macros.

EDIT: I don't have one, so I'm not sure how Thunder Breaker's linking works. If the skill-buttons need to be tapped repeatedly one after the other, I would suggest giving them the same treatment I suggested for Illium, where holding two different skill-keys down will chain them together, or they could just be in-game macro'd, where they would use the character's attack speed.