My bitch is currently in her second full estrus cycle...and yes..her vulva is certainly swollen and she tends to lick her genitals when she cycles....the blood droplets make it obvious of course...I have also noticed she tends to be more lovey-dovey when she is in heat....which others have told me is odd. I, like you, am waiting for her to reach maturity before spaying her.
For what it is worth....when my bitch is in heat...I do not go to the dog park unless I put her in a smaller area which I can guarantee is unoccupied.....and rare if I even go to the DP when she is in heat....I believe ( could be wrong) but the second to third week of the estrus cycle when the bitch is accepting to be impregnated would not be a time to take her off leash unless you control the environment.
I was under the impression heat cycles occur about every 6-8 months or so with a larger breed like a GSD.
I would say between the calendar and the dog licking herself accompanied with a swollen vulva is what tells me, loud and clear...my bitch is in heat.