op, just saw your post, have to go out soon and couldn't read and run (sorry haven't read all the thread)
This is so like my ds aged 9 and last week talking to some other mums of ds of same age (all at different schools) they were all complaining that their ds were the same. it made me feel so much better that others were struggling too. It isn't a recent thing with our ds, he has phases when it is better/worse.
Apparently they have a testoterone surge at this age and that causes a lot of anger. if they are bright and generally goodish at school, then they have to express that emotion somewhere and mum and dad take a battering.
A few tricks I have learnt (not that thye all work all of the time, but each one helps a bit)
Hunger and tiredness are a killer. Snacks are especially your friend. Some days ds comes home like a huge thunder cloud, as her eats the black cloud lifts
Put in place some house rules about whatever the key issues are. We have whole weekends ruined by the homework battle. So we have stopped. The new rule is, after breakfast on sat go to your room with homework. You can do it straight away or play with lego all day and then do it at 6pm, but you may not play downstairs/go visit friends til it is done. At 5pm (usual tv time) you may not go on tv computer til it is done. We will not argue with you about it, that is the rule, you choose how and when (if he needs help he is allowed to come and do it with me, but he has to ask)
When you have a house rule, then there is no debate (well, he thinks there is but there isn't) you shrug shoulders and say sorry it is the rule, and then ignore all that follows. If he is clever as you say, he very quickly realises that he has been outsmarted and it is down to him.
We have house rules over homework, swearing, hitting /hurting dds, smashing door/furniture. (eg swearing is immediate go to your room, and we will call him when he can come out reason- rest of family should not have to hear that language, so he is removed from rest of family)
Anger - we noticed that at times when he is angry, he is no longer able to make a decision, so if he is really wound up after school and you put in a sanction and he doesn't care, he may be beyond decision making (there is a biological change that happens when you are angry, the decision making part of the brain floods with a chemical and they become beyond choice) So when we see this happen, we take a deep breath and calmly (!) say go to your room, you are angry and you need to calm down. i will not discuss this until you are calm again. it can take an hour to clear the brain. Then when he calms down we take his concern seriously and talk about them, but mostly by then he says 'I was out of order and I amd sorry' at which point we have a big hug. There may be one consequence for his behaviour, but if we hadn't sent him up then there would have been a long string of consequences. This has really calmed everything down, and he understands it too.
But on the other side we have introduced Friday night time. The little ones go to bed and we have an hour which is his itme, he can choose. We have watched doctor who together (can't watch on sat eve as dds too young) and played a game. Mostly it is just the fact that he is allowed to be up and we are giving him this time which has made a difference. He also complained that because he reads to himself in bed, he wasn't getting a bedtime story, and he said he missed his 15 minutes of time. So now instead of bedtime story we have a short game/ cuddle and chat.
I have also consciously remembered to copliment him, tell him I love him, catch him out being kind/helpful/good and thanked him. Recognise hw is older and give him respnsibility. Ds is allowed to local shop and and library (veyr close) to his friend's house on his own. Let him know he can do it because you trust him.
and acknowledge that baby is a pain, annoying and not necessarily someone he loves, but as you all live in the house he has to respect baby's right to be in house too, so no shouting etc
sorry very long, hope something in there helps