Some sage advice above.
I think there are many possible steps between this and disruption, but the first step is just giving yourself grace. You have come this far and it is hard to find the words to explain to someone who doesn't get what it is like to live with a traumatised child just how wrung out and burnt out you can feel.
When you say you have done therapy, was this through the ASF and a private support clinic or the NHS? The only reason I ask is because CAMHS might be able to offer ongoing support, and pull together teams.
I would write a cold hard clinical list of all the incidents and go back to a GP and ask for an(other) urgent CAMHS referral. Pull as many safeguarding and mental health bells and whistles as you possibly can- he absconds, he physically assaults his sibling and you, he expresses violent thoughts, he refuses to follow instructions which puts him at significant risk of harm, so on and so forth, whatever is true. Count how many ACEs he has had and include that on the form, and ask for it to be triaged specifically by the LAC team. CAMHS don't require a diagnosis to work with them (at least in our area). We also said that being seen by a professional is good because other professionals are .ore likely to deal with them and take them seriously, or have referral pathways not open to you, so do go back to the GP and ask for onward referrals to as many people as you can.
Go to the school and note the same thing- what are they doing to support him? If the meltdowns happen after school then it is highly possible he is masking. Do they had interventions in place? Could they? Could you do a parental request for an EHCP?
If all else is falling, you could consider a fairly radical approach as you haven't lost anything if disruption is a viable option on the table for you.
For example, we have started incorporating significant elements of low demand parenting. Not completely, but his outbursts, aggression and frequency of meltdown have reduced in the couple of months we have been doing it. I follow atpeaceparenting on Insta and it helps me to contextualise things in his brain. I mix it up with therapeutic parenting, and it does help in the long run. There is something empowering to just allow some stuff once you can let to of the idea that they have 'won’. Things like I always allow him to win at games we play, we have a chilled approach to dinnertime, if he wants a snack we don't make him wait, and we don't limit it either. He is getting better at self limiting. We don't sweat the small stuff like please and thank you particularly when he is under stress. Within reason, he can control his own choices unless it is about health and hygiene or bedtime. Stupid stuff like whether he wants to wear PJs was causing us huge upsets. Now I let him decide, and suffer the consequences either way. He can't control others, but he does get to decide what to do with his own body as long as he is safe.
However, I always emphasise to professionals that this is a significant accomodation I am making, and I want it specifically noted that the level of control he requires is abnormal and debilitating. He is at significant risk of social exclusion, and requires continual monitoring. The professionals surrounding you should be made aware of this quite clearly, with a table of it spelled out if needed!
I don't know if you are part of adoption UK but they had a course that might work for you, it's about primary school age children and child on parent violence, it's on the 3rd December 9.30 to 3. I've not been to that particular one but something similar and the relief to have some strategies that actually worked was amazing.
I'm so sorry to hear about how hard it is, and I can offer a virtual handhold. Blocked care is so shit, it's like shamey shame mum guilt in reverse and on steroids and I get how horrible it feels. I'm sure you know all the places you can look for support, but just in case you didn't know, the NATP run an empathetic listening service, which might be useful.
It sounds like you have e been denied support that is desperately needed, but when things get this bad, sometimes bashing on doors is the only way to get heard. I'm sorry to hear how hard it is for you right now.