Lucy stopped growing at almost 2 years! I have measured her several times, yes, she is 74 cm to her shoulder blades. She started to grow wider since then, her croup is more muscular, her chest is wider, she is havier, I can judge even by photographs. She is oversized, but there's no HD, good God.
Your dog is a right size. Actually, for Agility they don't look for bigger, it happens that smaller GSDs are more agile and fidgety, exactly what the sport needed. Larger GSDs are bulkier, heavier and they are not so good at jumping, keeping the balance or squeezing between the posts. You will see that in Schutzhund there are very few big dogs too for the same reason. So, you should be full of sanguine hopes.