I think your doctor is a good one. He's not taking any chances, and that is the best kind of doctor to have. You aren't going to find women here who are going to agree with the "wait and see" approach, because we've all been diagnosed with breast cancer. Some doctors don't agree with that. They see no need to go through a surgical procedure for nothing. But a biopsy is the only way to be absolutely sure, and I'd rather have a biopsy with a benign finding than a mammogram in 6 months that said, "Oops... we were wrong." Honestly, most breast cancers tend to be slow growing, and so 6 months is not going to make a huge difference when a tumor has taken years to grow to the point that it can be seen on a mammogram. Phyllodes tumors are pretty rare, but you are right, they are fast growing. If they suspected a phylodes tumor, I'm sure they would have insisted on a biopsy asap.
I know it is easier said than done, but try not to worry til you know you have something to worry about. Worry won't change the results, and it really sounds like your doctor is saving you six more months of worry and anxiety.
You mentioned that you are peri-menopausal... are you taking hormones, by any chance?
Hope this helps!
"There's a difference between a philosophy and a bumper sticker." --Charles Schulz