Is my puppy under socialized?

My puppy is the most barky and reactive puppy in puppy class. My trainer suggested he is under socialized. I’m heartbroken because I thought I did so much with him and I don’t understand how this happened. In puppy class, he basically barks nonstop the first ~3-5 minutes of class before quieting down for the remainder of the 45 minutes (unless another puppy pulls towards him, or something new like a person comes into the space). He is 18 weeks old now and a shepherd.

Before he was 16 weeks old, we did: – 3-4 park visits, either being carried while walking around or one time sitting on a blanket. He saw lots of people, ducks, kids on the playground, kids on bikes, dogs playing or walking by, etc. – Home depot in the cart once – Sat outside the grocery store once – Vet 3 times – Carried him around my neighborhood twice, put him on leash on the ground in my neighborhood once – Sat in our building lobby once – Took him on the local train once – Puppy class every Saturday; not playing with other puppies but training on leash around them & around some adult dogs all on leash – Carried him around the mall (very busy with people) twice, put him on leash on the ground once – Has friends over 3 times – Went to the local sports fields and watched soccer games and people walking around twice – Sat outside the dog park in our trunk watching dogs twice; he started barking a lot though when dogs got too close to the car so we didn’t do this one again. – 3 different puppy play dates with older dogs – Carried him around a local farm/petting zoo type situation – Sat in the mall parking garage and observed once or twice

There might be some other things I’m forgetting. But basically since we brought him home at 9 weeks, he was out and observing the world 2-5 times per week until 16 weeks and above is all the things we did that I can think of off the top of my head. From 13-16 weeks it was a little less frequent as he was getting too big to carry.

Around 12 weeks he started barking a lot at things and hasn’t really stopped, even things he saw before. Today a bike came past us and despite seeing bikes before when he was 10-11 weeks old, he flipped out. His biggest triggers are dogs that are larger than him, he barks like CRAZY, and hearing other dogs barking also sets him off. Sometimes it will be random, like one day no issue with people walking around and the next he is losing it at every person (barking at them). When I take him on walks he definitely seems like he’s in “defensive mode” because he’s sniffing and scanning and his hackles are sometimes up if it’s a brand new area.

What more could I have done? I’ve read posts on here where people take their puppy out every single day multiple times a day and that’s how they make them bombproof. But I work 40 hours a week and wasn’t able to. I don’t know if I should’ve taken a few weeks off and taken him out more than I did. I really thought I did enough.