Op from what I've read this really sounds similar to my eldest ds from age 3-5, behaviour started with the arrival of a new sibling.
Interestingly though he didn't attend nursery for the first couple of months and when he did start it was morning nursery only, until he started school at 5, so I did spend a lot of time with him as I was a Sahm.
And from reading what youve written I think upon reflection this saved us as I think he needed time with me! It seems you are caught in a cycle of him craving your attention and displaying this through negative behaviour and you rebuffing it by spending very little time with him!
What I did was live my life around his routine. I needed to take him out a lot for classes and activities as he would become very mischievous at home and violent towards the baby. I had a couple of playpens for the baby if I needed to put him down but would mostly carry the baby around constantly, but subtly, trying to ignore baby and focus on ds. I would also complain about the baby needing things to my ds rather than talk about the baby needing special care, if you know what I mean 'oh no the baby is crying again- so much noise is hurting my ears!' He never responded to the typical advice about involving the child in nappy changing/ bathing as a little helper. He didn't like the baby so didn't want to help! I also breastfed and would just feed baby whenever ds was playing. I would arrange my days to get ds out of the house asap.
At night time I would sit with ds in the dark, listening to music and rubbing his back while breastfeeding baby. He would never be left alone to sleep as he would just become manic so i always stayed with him in pitch black darkness. No bedtime stories either as he would never listen or stay still for them. I could never understand why other parents would suggest this as it would never ever work for him!
Now he is 8 and he is a clever boy. No problems as school. He sometimes gets a mischievous look in his eye but I can reason with him more and can talk him down. Still don't do bedtime stories but we have moved onto audio books in the pitch black darkness, but I still often sit with him, to give him that special attention. I still put him to sleep first even though he is the oldest. I tell him I need to as he falls asleep faster than his siblings and he is the first up -which he loves as he loves putting on the TV first thing in the morning and being the boss of that! He siblings wake later. So that reasoning works for him. Also he falls asleep fast so then I can focus on the others who have a much more typical bedtime story etc routine dc.
But I think the key thing that I did (accidentally!) was giving him time and making him the priority (in his eyes) I don't think his siblings miss out as now he is more reasonable and has developed empathy for them (thank goodness!) they get along fairly well now too!
Not sure how you could turn this around. Maybe put your middle and even baby in nursery and reduce you ds' time there. Maybe make some disparaging comments about the baby sisters left at home with your friend and make him feel important?