To get the discussion back on track, I did a little analysis of my own, since I have some spread sheets from my own efforts at class analysis.
I too believe the monk is largely fine at levels 1-10. It's level 11+ where they start to get weird. I think looking at the subclasses will reveal what's going on here:
Open Hand, Tranquility: This is a free, very extended casting of a 1st level spell. But, since it ends early when you attack, it will only be beneficial in the first round of combat, and not offensively. This is hardly worth a Warlock Invocation, and needs a damage booster.
Shadow, Cloak of Shadows: This can grant advantage at the start of a fight, but that doesn’t even remotely equate to an extra attack’s worth of damage. Needs a damage booster.
Four Elements, Disciple of the Elements: Since these take an action, and cost ki, they only really get in the way of your other features. Tasha’s “ki-fueled attack” feature helps, but this needs a damage booster.
Long Death, Mastery of Death: This doesn’t increase damage round to round. It will help keep you up, though. This needs a damage booster.
Sun Soul, Searing Sunburst: For 1 ki and your action+bonus, you can make 4 attacks at 1d8+5ish. That’s about 38 damage. For 1 ki, you can throw a 4d6 sunburst, or 14 damage. You’d have to catch 3 enemies in the burst to exceed this damage. Very situational. For 2 ki, I almost think you’re better off using Searing Arc Strike if you’re fighting one major target with multiple allies. This doesn’t feel like an overall damage improvement, just a different option, so the subclass still needs a damage buff.
Drunken Master, Drunkard's Luck: This will increase damage a bit, but not equivalent to a whole attack. This can still use a damage booster.
Kensei, Sharpen the Blade: This is a cheap magic weapon spell, which would normally cost 3, 6, or 9 spell points and have concentration, but it only lasts a minute instead of an hour. It’s just a new “spell” to spend ki on, so not a real damage boost, though it will boost short term damage. The subclass could still use a real damage boost.
Mercy, Flurry of Healing and Harm: Finally a true damage boost! This adds damage nearly equivalent to 1 attack (Martial Arts Die + Wis mod, instead of + Dex and potential unarmed enhancement bonuses) that’s potentially added as a rider once to up to 2 attacks (if you use both attacks with a flurry, your chance of landing this “extra attack” goes up).
Astral Self, Body of the Astral Self: Another damage boost. This extra damage is very similar to Mercy’s, but instead of having Wisdom mod it benefits from unarmed enhancement bonuses and it benefits you even if you aren’t using flurry (and you have up to 4 chances to ‘land’ it).
Now, both Mercy and Astral Self have portions of their 11th level features that aren't just damage. As such, I think we could 'afford' to simply go through and add a damage feature to each of the other Monk Traditions. This damage should be on the order of a single attack, but not quite as universally applicable (the fighter is the only one who gets such universality).
Here's some quick thoughts:
Open Hand, Improved Flurry of Blows: Add 1 extra unarmed attack to martial arts, and flurry is now 3 unarmed attacks. Pure and simple. The miss chances, instead of 1/round bonus damage if one attack hits, lessens it over Mercy.
Shadow, Shadow Strike: Give them some sneak attack. Once per turn, you can deal extra damage equal to your martial arts die to one creature you hit with an attack if you have advantage on the attack roll. You don't need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll.
Four Elements, Fist of the Elements: I use a fully rewritten way of four elements monk, but here's a quick fix. When you use flurry of blows, you deal extra cold, electricity, fire, or thunder damage (your choice) on your unarmed attacks equal to your Wisdom modifier.
Long Death, Death's Touch: When you use flurry of blows, you deal extra necrotic damage on your unarmed attacks equal to your Wisdom modifier.
Sun Soul, Searing Fist: When you use flurry of blows, you deal extra fire or radiant damage (your choice) on your unarmed attacks equal to your Wisdom modifier.
Drunken Master, added to Drunkard's Luck: Once per turn, when you miss with an attack, you can make an additional unarmed attack at advantage.
Kensei: really tempted to just give them extra attack (2), but only when using sharpen the blade.