"They need your love the most when they deserve it the least" is what someone said to me about DS in a similar situation.
The punching/smashing thing being directed at you - it's partly separation anxiety. Or it was with DS. It was like he was afraid of rejection so he would do the things more likely to make him be rejected. Testing the boundaries, I guess? It seemed to be tied up with security issues, anyway.
Things that helped - reassurance he was loved even when he was being awful. The "angry tent" - a pop-up tent we put cushions and toys in which he could go to and lash out and hit things safely. Sometimes he would have to be put in the angry tent, as that's the place we go to be angry and hit things - but after a while he would voluntarily go there and let off steam. "I love you but I think you need to go calm down on your own or be angry in your angry tent. Then we can talk".
Hugs. If offered at the wrong point, it could make things worse. So something along the lines of "If you're angry right now, you probably don't want a hug. But if you finish being angry and you want a hug, I've got a really big one waiting for you." Most of the time his outbursts would leave him exhausted and weepy and in need of a hug. I think he freaked himself out by how angry he got sometimes.
"Drawing a line". Sometimes you could tell he'd backed himself into a corner and couldn't see how to back down without losing face, and he wouldn't back down. So sometimes saying "Shall we start over, and just draw a line and forget this happened?" Pretending to "rewind" or drawing a vertical line in the air with a finger would sometimes help. Possibly because it was a bit silly.
Talking to him afterwards about why he got so upset (once he was calm again) and what else he could do next time he feels himself getting cross that might be a better way of dealing with whatever it was. Telling me he was starting to feel cross, taking deep breaths, or whatever. Then the next time he started kicking off there was sometimes a brief window where things could be headed off. Me acknowledging that he was feeling cross, and telling him I can see he was trying really hard not to lose his temper.
None of these worked 100% all of the time but it felt better for us than the cycle of him getting angry and being met with anger for his unacceptable behaviour, him getting angrier still... He still has his moments of anger but no violence any more. It has definitely been a process rather than a quick-fix.