8 week old puppy is extremely reactive

I recently got an 8 week old puppy, I was prepared and ready I have been planning this for years now. Not my first puppy either I have a very well behaved German Shepherd and Rottweiler. I’m aware of how to properly socialize a puppy and not let every single person or dog interact with them so i went to a parking lot that was moderately busy opened up my tailgate and took his frozen meal back there and we were just going to sit and people watch. Every single door that would shut he would bark and growl every person that would walk by in the distance he would bark and growl. When he’s doing this he’s not cowering or tail tucking. His body is rigid, tail straight in the air and growling/ barking like no tomorrow. At 8 weeks old I’m extremely concerned with his behavior. We have done a couple sessions of this now and he’s the same way every-time. I’m worried this isn’t something he’s just going to “grow out of” I play desensitizing noises at home and he has structure and outlets at home as well. I’m at a loss on what to do as I had big plans for this puppy and now at 8 weeks old he’s extremely reactive. He came from a good breeder and back ground. I’m just at a loss.