There is enough evidence to have persuaded me that for every season a ****** is allowed to have, the RISK of developing cancer later on, increases. But this is only a risk. It doesn't mean that the ****** who isn't spayed until later on absolutely will develop cancer. Almost all our bitches were spayed later on, on retirement (by around 5) but only one developed mammary cancer when she was in her 13th year. There is also the risk of an entire ****** picking up a uterine infection such as a pyometra, but again, it is only a risk, and we never had a pyo in any of our bitches, in how-many years/bitches.
My advice isn't GSD specific but I'd suggest the incidence of cancer in the later-spayed ****** isn't breed specific either. However, I would say that perhaps you might allow her to have one season - my vet advised me to do this with my Whippet when I asked him to spay around 6 months, suggesting she could have bone growth complications if spayed early, and further that bitches spayed early sometimes develop spay incontinence. I wasn't ready for that response, and waited. However, she'd not come into season by 11 months so I asked him to go ahead and spay and she's now 3, and fine. So far!!