I dont think it ever hurts to mix it up a bit. Playing someone with a predictable serve is actually a bad strategy on their part as you are likely to know where the ball will go and how it will bounce.
I have played many hard hitters and they often thrive off a hard hit serve by taking its force and thumping it back.
I would try and add a bit of spin to it though, just to take it away a bit, if you know what I mean. But you dont need to add more power. Its like golf really, if you let your swing do the work and by slowing it down you have more accuracy to hit the corners or lines then go for it.
I think against any opponent you need to be adaptable and able to mix things up if you need to and exploit their weeknesses.
"Tennis is just chess with balls."
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