You can say it softly and still mean it completely. Volume isn’t the same as firmness.

Force and firmness get confused because loud people are often trying to seem firm. A quiet sentence that does not move is considerably harder to argue with than a raised voice, partly because there is nothing in it to push against. Volume hands the other person something to react to instead of your actual point.

Say the difficult thing this week in your ordinary voice and then add nothing. Repeat it once, unchanged, if it gets talked over. Steadiness is what carries the weight, and steadiness is easier to hold quietly.