A boundary said kindly is still a boundary. Volume isn’t what makes it real.

There is a belief that a limit needs force behind it to count, which is why people wait until they are angry enough to say one. The firmness lives in the content, though, not the volume. Said calmly it is actually harder to argue with, because there is no tone for anybody to react to and nothing to accuse you of. All that is left is the thing you said.

Say the next one in your ordinary voice. Keep the content exactly as strict as you meant and let the delivery be mild. If somebody treats the kindness as an opening, repeat the same sentence and add nothing.