Struggling bad with upcoming 5 year old.

[HappySamii2826]

Right, this is a bit of a long story.

So, I'm going back to when he was maybe 2.5, he's always been a naughty child, always extra to deal with. We used to have 2 cats, and he always used to hurt them and do cruel things. Smothering them with pillows, punching/smacking/kicking them. Hurting me. Fast forward a little...
He has a best friend macie, my best friends daughter. She's 1 year younger than him and he's alwaus hurting her. He's pushed her down the stairs, squeezes her arm all the time, leaving red marks all over her. Twisting her arm. Again, blah blah.
He has a baby brother, 1 years old. He used to sneak out of his bedroom in the middle of the night when we was asleep and come into our room, hurt the baby when he was a couple months old. (Guessing jealousy). 5yr old still hurts baby now. I literally feel like I can't leave the two together for max 5 minutes. And that's impossible as the dads at work, and of course, mothers need to go and wash up, cook.. the endless list.
The other day, the baby was standing on the floor holding onto the armchair, and the 5 yr old was picking him up by 1 leg and yanking him. Poor baby was upside down crying like mad. I sent him to his room, of course I shouted at him and explained that's a horrible thing to do, should love your brother blah blah.
But honestly, in all cases where he's naughty, I go through all the steps, naughty step, chart, taking things off him. No sweets, bed early if he continues.. nothing works. I tell him off, he smirks at me. He looks right through me. And very rarely when he does seem to listen, he has his time out, 4 minutes later he's done it again. It's like he likes being naughty. It literally does my head in. Me and the dad are struggling what to do.
He's always breaking things, doesn't matter what it is how durable it seems to be, he'll find a way. It seems impossible for him to it still, full of energy, I do take him out, even a trip to the park doesn't chill him out. He never ever ever ever listens.
He's in reception now, started about a month ago, almost every day I pick him up, the teacher calls me over explaining what he's done that's naughty, they also have a special chart for his behaviour, almost the same as I have at home. Since starting school, he's also wetting himself daily. His back chat is absolutely insane, he never stops doing it. Hell be sent to his room, he'll be banging the doors, kicking the walls, shouting through the door with a demonic scream. And this doesn't last a little while. It lasts HOURS!
I've been to the doctors about my struggles with his behaviour, I've been fobbed off. Of course I have spoken to the school as well, I should be starting a parenting class in january, I've completed a form for action for children. In all honestly I'd like an assessment done for him. I could be wrong as he is my forst chilf but I feel like he's not exactly a "normal" child.
I'm scared what he could potentially do to the baby, there's no chance in hell that they will be sharing a room. (Were in a 2 bedroom flat) so the baby will be having to share our room until we finally move out.

Honestly, any advice anyone could give will be taken happily. Just feel like there's no light at the end of the tunnel and it's getting me down. Questioning my parenting, I struggle to sleep because all night I'm just thinking over and over what to do but I feel like we've literally tried everything.

I've probably missed parts out btw, this is just top of my head the main struggles.