It is a really tough age. I feel like everyone goes on about 2-3 year olds and I am like
because my two year old is a dream. The occasional tantrum lasting less than five minutes. My five year old is a volcano. Always ready to explode at the slightest provocation. I am constantly having to take headache tablets to ease a headache caused by him screaming at me. It was my eldest's toughest age as well (and he is now a teenager!!)
What I think I would do is identify two issues and really hone in on those specifically - if two is too many, pick one. Everything else just go for the path of least resistance and don't worry about changing it right now. You can go for those later.
From your post, it sounds like the two biggest issues are dressing himself and not hurting/upsetting siblings.
You said that he isn't interested in a reward chart - my guess is that either the steps to obtain the reward were too big which means that they feel unattainable, or it's being framed as this optional extra but you're trying to use it as a threat e.g. if you do that/don't do that, you won't get your reward!
So you could do one of two things with this (maybe both?)
One is to make the reward something that he currently gets but could be framed as a loss of privilege if he were to lose it - making getting the thing which is currently unconditional, conditional on certain behaviour. That could be pocket money, TV time, etc. It needs to be something that is easy for you to control and which isn't difficult to implement for the others. For example if the others get TV time and not him that would be tricky, but you could restrict/allow access to something that is only his, for example if he has his own tablet.
The other is to look at what he has to do to get the sticker/points/reward and break it down much smaller. If that means that you would be giving out the reward too much then you can use points or stickers and X number of collected points leads to the reward.
You can break down a task into times or you can break it down into steps or both. So for example, for getting dressed you could break it down into morning, before school, after school and bedtime and just start with the one which you currently have the most success with. Once you're being successful with that, increase to include the second time, or make a chart with the 4 times listed and he needs to get a sticker in 3/4 to get the reward.
For steps you could break it down like by item of clothing. Or for the being nice to siblings issue you could look at improvements e.g. not being physical (but ignore shouting etc) and then when that is better then you can look at shouting and change that to talking (but ignore rude phrasing/demanding) then work on better communication.
This is also really really excellent and will really help you immediately. It's very well designed and offers skills that build on each other one by one. If possible, complete each module together with your DH and then practice before moving on to the next. But it works when just one parent does it too.
https://www.coursera.org/learn/everyday-parenting
Beware of the assumption that "he has done it before so I know that he can do it". There is a difference between being able to do something physically, and being so well practised at it that you can do it even when stressed, tired, overwhelmed, frustrated etc. He might need more practice at the component skills or there might be some totally unrelated stressor which can be reduced to help give him more mental bandwidth (if you like) to meet the same expectation all the time.