Hi
hear you there ! I guess it's pretty much the same for every pup. At home everything good, as the stimulation are low, but once outside, nothing else matters (yeah love Metallica :))
I think one of the key here is to have the most high value treat ever. It changed much when we switch : normal education low value treat, he wouldn't bother to even look at us, but when outside we switch to dry lever, of cooked chicken, rest of our beef steak, and then we managed to get at least a little gaze here and there, or some good recalls
Also, I think if your training environment are House VS Outside around dogs, the difference can be too much to handle. If you have a yard, train him in the yard;
Or if you can, set a puppy playdate : that way you can train with only one dog he knows as stimulation : it's easier than outside where there are dogs, new smells, and all at once...
Training with little increase in stimulation would be more manageable
We also attend puppy school : it's a good environement, supervised by professionals, where the stimulation can be controlled, and everyone is here for the same reasons.
just a little self experience, hope that may help