u/Akiramenaiii

Hello everyone, I really need some advice as I don't know what to do anymore.
My big boy, Noctis, is almost 11 years old, we've had him since he was 18 months old. Back then, we adopted him and another cat together, and they got along really well.
Unfortunately, our other boy passed away from FIP close to a year after we adopted them.
All of us were heartbroken, and a few months later, we adopted another cat, Neji, who needed a new home.
They got along... okay-ish. Our first two liked rough play and used to chase each other across the house. Neji does not like that, not because he isn't playful, he just prefers to play by himself with toys or with us.
They were never the best of friends, but they tolerated each other.
In 2022, I gave birth to our first child, and unfortunately, Noctis' behavior went downhill from then on.
I will spare you the exact timeline and just give you the current situation:
We feel like he doesn't want to be here with us anymore. He can spend HOURS roaming the apartment, meowing at the top of his lungs, and no matter what we do, he doesn't stop. He is obsessed with food (he has always been) and will chew through garbage bags to get to a crumb of something inside.
He attacks Neji multiple times a day, to the point that they both permanently got scratched up noses. Neji screams every time they even exist on the same plane of being. Like, the fighting is BAD. Neji screams like Noctis is about to kill him, and it has woken us and our son up multiple times. 10 minutes later, they can stand next to each other like total angels, begging for food together.
I'm the only person Noctis tolerates in some way. I'm the only one who can pick him up and pet him, as he hisses and growls at everyone else. Sometimes, when he's in an especially foul mood, I also get that treatment.
When he's in a good mood (rarely), he comes to me and wants pets (not cuddles, he doesn't like being picked up, never has), and then he seems thrilled, and I get to pet him for about 10 minutes before he's done and leaves again.

The problem is mainly the fighting. It has led to our son growing up with the notion that cats always need reprimanding and that living with cats is stressful and scary (because of Neji's screaming).
We have done everything I could think of to help Noctis adapt. We've gone to the vet multiple times to check if anything was bothering him or if he was in pain. Nothing. He gets enough food (he is a big boy, and has his ideal weight). I also feed him different things every day to make sure he doesn't get bored with his food. We try to give him attention whenever he wants it, but he doesn't want it much of the time. When we play with him, he does it for 5 minutes, and then he leaves. He comes to bed with us for 5 minutes as long as nobody moves, and then he runs away to meow. We gave him multiple places to retreat to if he wants, and he uses some of them on and off, but mainly prefers to lie alone somewhere in the open (or in a box, loves them) and sleep there.
Enrichment toys only interest him as long as there are treats inside.
Maybe he is overstimulated or feels like there are too many people living in the same space now and wants to go outside, even though he is an indoor cat and has always been. He got out once (it was some Houdini type shit, I tell you), was gone for three days, and then just came back home. We can't let him out, though, as it's not safe. The balcony is the best I can do, it is completely cat safe and he spends a lot of time there.

I... don't know what to do :( We've had him for so long and love him, but it is becoming more and more difficult every week. I feel like he is not happy anymore. But rehoming after almost 10 years also feels wrong. If you got anything that may help, please let me know.