Op apologies if these things have been suggested or mentioned already but wanted to add some additional thoughts.
To me,.especially the car scratching thing sounds like he is pushing boundaries and trying to see what reaction he can get. Because the behaviour has continued he is pushing more and more by increasing the behaviour.
Have you tried watching supernanny or little angels or the house of tiny tearaways. These were on about 10 years ago but focussed on families with children that had behavioural issues Including adhd. You should get them on you tube but may help you to understand what is going on in your little boys head and ways to deal with it.
There is an element of ignoring some bad behaviour so he doesn't think it bothers you but when it gets to be verging on dangerous, nasty or violent then punishment is introduced but it needs consistency, so not giving up after a few tries but to keep doing it over and over (you will need your happy place)
I had to do this with my dd who wouldn't sleep without me in the room and would keep getting up. I had to just keep putting her down and sitting on the floor, over and over and over. I would sit with my hands in my head and count. It was hard and relentless but after about 3 weeks she learnt that it wasn't getting her anywhere and she self settles really well now.
My dd laughs in my face when I tell her off and it is so hard to keep my cool so I a trying very very hard but it does test your patience
When you tell him off do you get to his eye level and make him look at you. And when you take him off the naughty step do you ask him why he thinks he is there and if he doesn't answer properly do you leave him there longer to think about his actions? Or start the time again if he gets down or argues?
Does your 2 year old get punished if she mid behaves? If she does something and isn't punished he may consider unjustness and have a resentment.
Also when he is mean to the girls who you deal with first? Do you tell him off or check the girls? Might be worth trying it the other way around for a while to see if it has an impact.
I know you are bf and that is Amazing but have you considered changing to bottles for your own sanity?
I would also try to find a child psychologist or behavioural specialist that can monitor you in the home (you need to behave normally) to try and help develop some techniques.
You may also want to consider counselling yourself. It's great for people even without a diagnosed mental health problem as it's way of venting your feelings without worrying about someone judging you or jumping in with some helpful advice but rather you finding ways to deal with your emotions yourself and understanding why you might feel how you do.
It's going to take time, don't forget these little humans aren't born with functioning filters or boundaries, they are constantly learning and we are there to teach, even if we don't like teaching every minute of the day. Alot of behaviours are learnt but they can be retaught everyone has the ability to change with guidance and support.
Try and take some time for yourself!
Oh and thoughts on nursery, there is a whole theory on pack mentality, my dd is a different person at nursery and I believe it's because she is doing what everyone else does.
You can do this op.