Ki Resonance might be better than you think

I have noticed when discussing monk with others that most people multiclass away immediately after level 6 because the higher level abilities don't look appealing when you read the descriptions. I'm here to encourage you to consider at least *trying* 9 monk, or higher.

Ki Resonation: Punch is a level 9 Open Hand monk ability. These are otherwise normal unarmed attacks and unarmed bonus action attacks that apply Ki Resonance to a target for 10 rounds. There is no Ki cost associated with these attacks, so you can basically just replace your "normal punch" with these.

Ki Detonation - Costs 1 Ki point - this is a free action meaning it does not cost an action point or a bonus action point, and can be used any time in your turn even when you are completely out of actions.

When you choose a target to detonate with Ki Detonation, that target and any other enemies within 5m will take 3d6 force damage, and if any other enemies within 5m are also affected by Ki Resonance, they will also explode for 3d6 force damage in a 5m radius. This damage does not affect friendly targets.

If you also take the Mobility feat, it becomes very easy to use your actions in a turn to tag every enemy in a small area without receiving attacks of opportunity, then detonate them all at the end of your turn. You can of course use multiple turns to setup larger explosions since the condition it applies lasts for 10 turns.

If they are more spread out, you can apply Resonance to as many targets as you can in a turn or surprise round, then have another character use Black Hole to pull enemies together before you detonate them with a free action.

Even without the Mobility feat, it's very often that you can be surrounded by 2 or more enemies that are easy to land a hit on within your turn, and spending a single ki point to detonate 2 or more enemies in proximity can be an efficient use of Ki points since the damage is AOE. Using it on just two enemies applies 6d6 to each of them, which is a bit less than a flurry of blows against a single target. More than two and it gets much more efficient.

Damage taken by Ki Resonance can also trigger the tadpole power Cull the Weak which can lead to some pretty impressive chain reactions.

We must acknowledge there is an opportunity cost for going more levels in monk to get this ability. I am not trying to claim that this is universally better than your multiclass build. Maybe you only get 3 thief and can't add fighter levels or barb levels, but in exchange you get more ki points and Resonance. Maybe you go full monk and grab a 3rd feat like Alert so you can always go first and have 13 Ki points to play with each short rest.

I just want to provide you with more information about how these abilities work in practice since the descriptions make it seem so weak and make you think that you'll have to spend actions to detonate.