They focused on AJ so much to show you that him and Tony are both a like. It’s their upbringing that’s different.
On my most recent rewatch I actually came away thinking AJ probably has the most subtle but well written arcs.
S1: little brat who tries to be a cool kid finds out his dad is a mobster. He thinks his dad is invincible but he ends up getting shot. He's actually around for his dads goons to discuss payback.
Never gets sit down properly to be told the whole truth, except by his sister who has her own bias.
S2: now a teen who doesn't understand why his grandma and uncle are suddenly outcast. Gets told what to think/how to act different by his mom/dad/sister/grandma/friends again
without being told the truth about anything. Starts to lose faith in the school system.
His godfather (puss) will disappear from his life and he won't be told what happened (can assume his sister will hint at it, his parents will talk around it. Shyt his grandma might tell him but in her own twisted way). His grandma also spends the season telling him life is pointless.
S3: grandma dies. Loses his way but his sisters boyfriend shows up and he kind of idolizes him....
is in the room when Tony/Carmela find out Jackie Jr got shot to death in a drug deal gone bad. Instead of talking to him like a normal father his dad uses it to guilt trip him about fukking up at school. He's also the first one to actually tell his sister about it. She freaks out and he doesn't seem to understand how he should be reacting to it --> easily top 5 most important scene in AJ's arc. Another key scene: when Tony is told, in Pine Barrens, that the guys fukked up he openly talks about how he might get merked visiting the Russian now. AJ IS IN THE BACKGROUND and visibly reacts to this. When Tony looks at him they both shrug it off and Tony walks away. Another top 5 essential AJ scene. S3 is where he really gets fukked up psychologically, imo.
S4: is the only one around to fully witness the ups and downs of his parents relationship. They BOTH use him to sneak around on each other. Carmela actually uses him to visit Furio and he seems to kind of understand what she's doing but not why she's doing it (in the way he tells Meadow about it).
Again key to his lack of understanding people's motivations and relationship to him. Plus just total scumbaggery on both his parents parts. Furio will suddenly disappear from his life and he will not be told why. Based on his parents fights he might hear about it though. Ralph will disappear from his life.
I dont remember in which season he drops a plate, and Tony goes "you want me to get snipped with this as my male heir 
"
S5: his lack of outlook on life fully exposes as he knows 0 ideas or ambitions on universities at this point. He also starts using his dads rep to get ahead and gets better at manipulating his parents. He has 0 respect for his mother,although he seems to understand that his dad's not all it either and he's genuinely happy that they get back together. I think if they stayed separated his breakdown would've happened sooner.
Adriana will disappear from his life.
S6: dad gets shot by his uncle. He thinks it's on him to get retribution but fails to do so, and his dad tells him he's not about that life (so again, he loses another road to follow). Vito will disappear from his life. His girlfriend is richer than him but lives legit which he can't handle and his friends hang out with him because of his last name and because they think he has money. Meets that Latina and gets a taste of a life that he could want to live but she doesn't respect him and leaves him as well. His other "friends" ARE about that life so he feels double worthless. Christopher dies but he doesn’t see his dad genuinely grieving at all. In his total melancholy he thinks suicide is a romantic/deep thing to do but as soon as he gets a taste he regrets it and fights it.
Wants to be deep by joining the army but gets sold IMMEDIATELY by the promise of a en empty career in Hollywood. What's dope is he actually for a second realizes his dad is bullshytting him ("Uncle Carmine makes porn
") but he still buys into it.
I see AJ going two ways at this point:
1. Tony lives but gets RICO'd: without his father's backing I think he'll get treated like absolute dogshyt in Hollywood and knowing him I think he'll quit and either end up back with his mother or he'll find some woman who will use him. Either way he'll be miserable and most likely an addict.
2. Tony gets popped in front of him: I genuinely think this will drive him insane and he'll become an addict as well who lives with his mother or sister for the rest of his life, which might not be long.
And this is me freestyling his story and missing some beats. I think the actor wasn't always the greatest but he was good when he needed to be. And the story wasn't always the most exciting but if you lay it out like this I respect the fukk out of it. Until the end they never really made his stories dominate the rest, but the path was laid down through a lot of subtle scenes in which he simply was in the background or quiet.
GOAT show when this is one of the less exciting characters/stories
as where Michael is more reserved