Might be worth adding the time to learn a kick serve. More variety will keep your serve less predictable and draw more errors for your partner.And as I said, I have one serve. One. No kick, no true spin serve, no flat serve. It has one speed, not slower not faster. One serve that I have managed to get pretty darn decent and can place. That is it.
Another thought would be using signals, although I find this can throw some people off. But it can give volley confidence since they know where the serve is going.