Yeah I understand, never doubted you are a fantastic owner and Cargo is one lucky doggie - it comes across in all your posts. Just wanted to let you know about some of the dangers of dog parks.
You do need to have a level of 'trust' when training recall but there are certainly safer environments to test in than a dog park imo. It's not just your dog you have to worry about, it's everyone elses. You can have a dog be fine with the first 100 dogs it meets, and for some unknown reason has a problem with number 101. Or another dog could start something. Or a small dogs owner could throw a fit for no reason and cause problems for you.
It's not fair the stigma that dogs like rotties, bull breeds, dobes etc carry around but that is the reality. The sad thing is that if there ever was a dog fight, if the dog was smaller or a breed percieved as more "friendly" (ie a lab or something), the bull breed/rottie/larger dog is always going to be the one looked negatively on, even if it wasn't the one at fault.
I have a 'bull breed' dog and I am acutely aware that if she was ever in a disagreement with another dog the blame would be put on her. She carries the stigma everywhere she goes. Even at dog training, she was sitting in line quietly and a very small dog in the next line was going off it's head barking and lunging at my dog. Some woman says "careful or you'll be eaten!" directed towards the smaller dog, implying my dog would eat it. Even though my dog was sitting there not giving a damn.
I totally get it.. He does get a bit ... snappy every now and again, and i think that's my fault.. i didnt socialise him when he was a pup- i kept him away from other animals cos he was so small, i was scared other dogs would attack him cos he was only a pup.. then when he got big, i was scared he'd hurt another animal.. he doesnt like dogs bigger than him, he gets a big ego when bigger dogs are around- just walking passed them in the streets he tries to get them, i think he tries to make a point that just cos he's smaller- doesnt mean he's weak.. and it's really frustrating

luckily, there's not to many dogs who are bigger than him that ive seen haha..
I had cargo at the shops with me, and i was sitting on a bench and he was sitting on the bench next to me, and a mum grabbed her small daughter and was like "dont look at that dog, they get lock jaw" i was like ... since when do rottweilers get lock jaw you uneducated little mole!!
but then there's the really good ones, a little girl came up and she was with her mum and she was like "can i pat your doggy" i was like

yeessss please haha.. and she said to me "i have to let him smell me, so he doesnt get frightened" i said to her mum, she's raised her well to let a dog smell you first and not just go straight into it- she said they had rottweilers at home