D&D 5E Monsters charming PCs during combat

[Greenstone.Walker]

2. Wait, are you saying the spell slot is lost if the trigger doesn't happen within one round? We haven't been playing that way.

Yes, the spell slot is spent when the spell is readied, as are all of its components.

You also might not have noticed that readying a spell requires concentration. That bard won't be using bless or cause fear or anything else that requires concentration.

I would be dealing with the situation by making sure that the bard's player clearly specifies the trigger for the readied action. What percievable action are they waiting for?

More generally, charm magic is a situation where the game is taking away some agency from players. However, this is being done with the players' permission (they chose to play the game knowing that such magic exists).

If the players aren't happy with this (something that might come up during a session zero) then there is nothing wrong with removing such effects from the game. Just make sure you remove them for everyone. If the foes can't charm the characters then the characters can't charm the foes.