To be honest in the heat of battle I wasn't really looking at the HUD; I needed to look at the enemies onscreen 90% of the time. The only thing on the HUD I was glancing at was the main icon of which charms I had in each hand and the timers for when each would expire. I didn't have the time to memorize the meaning of each icon and remind myself midbattle which charms had what effects.
To be honest, I think the problem is just the fact that the third charm I acquired, the one with a boot icon on it, had both a main effect AND a special effect when it was thrown that felt too hard to remember. I think if each charm had only a unique main ability and a unique passive effect, as they already do, then that would be enough complexity for the charms. Passive effects aren't a problem for complexity as you don't really need to pay attention to passive effects for them to be useful, so its easy to just not factor them into your strategic thinking and still play pretty optimally.
To be clear, I think having an effect for when you throw charms is a great idea, as it creates interesting decisions for how you best utilize your charms, but I don't see why the effect needs to be different for each charm; that's where it becomes too hard to remember. Maybe you could simply have every charm do the same effect when you throw it. Throwing the boot knocked enemies back a bit, I think if every charm did that it would be a consistent option that would be easy to remember even in the heat of combat. It would still create interesting decisions, because if you have two charms with different effects and need to knock enemies back, you have to choose which charm effect you are willing to throw away and which you want to keep. That's just my idea though, I don't know if that fits your design plan for the game.