RE Suppression method
At 230 V, 20 A, and above the limit by 10 dB, you don't just slap on a cap somewhere.  You'll need a serious line filter at least, and that's just a bandaid instead of a cure at best.

The real problem is lack of attention to emissions thru the whole design process.  You don't just build something, then test it, then poke around sticking filters on until it passes.  It would take a large, heavy, and expensive filter to fix the problem after the fact like this.  The fact that this is all you're asking about shows that lack of proper design up front is the real problem.

So the right answer is go back and design it right, this time actually thinking about proper grounding, minimizing high frequency loop currents, etc.  There is no good way out of the mess you're in.  Good design costs real money, but now you can see how bad design costs even more.