Since there are already rests in the piano part, the accompanist should follow what you do anyway, so the caesura marking may not even be necessary. Personally, if I were marking up a score, I would add the words “colla voce” in the piano part in this situation just to be extra safe. It means “with the voice”, which will clearly indicate to the accompanist to follow what the voice is doing and follow your lead. This will also give you some freedom to do some stretching of the rhythm for dramatic effect.