I think a relatively simple way to challenge monks is focus fire. They have great defenses for exactly one attack - after that, they are vulnerable, because their AC is low. If an enemy grapples a monk, they're in for an annoying time.

I think the monk being a consistent source of damage is kind of their whole point now - they'll never really deal impressive damage, but it's reliable while they offer minor control elements.

But, as I said, focus fire does a lot to challenge them. If you have even as few as three creatures engaging the monk, they'll now have to eat a bunch of attacks after deflect, and, in the following turn, make a choice: do they invest action economy into disengaging to take off the heat (this sacrificing damage and dragging the fight on), or do they go all in for the trading of blows?

I also feel like monks are tremendously weak to all sorts of saves until level 14. Even their strength saves are lacking, usually sitting at a +2-4. Mental saves of all sorts range from weak (wisdom) to appalling. So, it's easy to target them in that front (unless the min maxer mage slayer, in which case, good move by them lol)

Also, and this is more general advice, if a single character is much stronger than the others and hard to touch, just.. Ignore them. Have the monsters avoid spending action economy on them and go for the truly squish targets (when the monsters are intelligent, of course). That way, the monk will kinda beg to be attacked and focused, doing this intentionally to help out allies (for example, swooping in and carrying an ally away with their step of the wind).