14 weeks is good timing imo. Often younger puppies tire before the end of the session. Also when they are younger, they are still very focused on you so don't get as much out of the learning to focus on you while other things are going on. They can barely notice the other dogs sometimes, where's an older pup will be actively gaining something from ignoring and working round other pups
Like having a young toddler, sometimes when they are younger it's easy to get lulled into a false sense of security that they don't run away, follow your every worc etc but actually as they get older, less interested in you they become more willful! A 18 month old toddler, and 3 year old would get different things from going to a group with other kids.
If it's an imdt one I'd expect it would go well with the easy peasy book. Check it's a positive trainer rather than balanced, and that they are properly qualified with imdt
We found the classes really helpful, and continued as he got older into adolescent classes
I agree that the ideas often aren't massively new, but it helped having a real structure to follow, and to focus on certain things. Having a puppy is so overwhelming because you're trying to teach them everything to succeed all at once. It was helpful to be focused on a smaller number of set things each week. Doing similar tasks each week also meant that we could mark his progress even when it felt like things weren't progressing or he was having a week where it felt like he's forgotten everything he's ever known
Our trainer was also able to point out a couple of things we doing wrong eg confusing hand signals, marking at the wrong time that encouraged breaking of stay. She was also able to reassure us that we were doing the right thing and to preserve even when it felt doomed! She pointed out our progress, made us feel more hopeful at times and ultimately was generally very correct!
It's also helpful to have other people in the same boat, and to watch other dogs actively go through the same things your dog is eg, watching a fear stage etc, suddenly getting up at 5am etc
Mostly though we benefited from the routine. It meant we practised in a structured way even when we felt too busy with life, and the fear of being exposed for blatently not practising in class kept us on track!
It's a bit like a slimming club, the ideas you probably know and you can do it at home. However it's also helpful to have external advice at times to point out where your going wrong, A group to get ideas from bounce off and support each other, accountability with a weekly meeting and a structure to follow