OP I think it was disingenuous of you to post this in AIBU 'because all of us should know and be angry at the state of SEN education' without mentioning until another poster asked pages and pages in, that your DS is adopted. As an adopter you have access to sources of support that most parents of DC with additional needs can't access. You can get the head of the virtual school for LAC in your area fighting your corner. You can suggest school use some of his enhanced Pupil Premium to pay for LSA time, or his private therapy for that matter. You could access the adoption support fund to pay for the private assessments, therapy etc that you have to work to pay for. You could request an assessment for an adoption allowance to cover the cost of leaving or changing work or taking a sabbatical. You could even go nuclear and say things like 'placement at risk of disruption' to get SS hopping. You are in a better position with more recourse to help than most parents of DC with SEN, yet you are responding here like you are the only parent having to live this shit.
I am a single adopter of 2, both mine are in mainstream primary and both have additional needs. So I do get how shit it can be. But I also get that being adopted means my DC get enhanced Pupil Premium, which can cover some of the things they need to cope in ms school. For context, the £ of my 2's PP is double the school's entire English budget for a year. We are more fortunate than most families with SEN DC to be able to access this.
We are also fortunate that I get an adoption allowance, and higher rate DLA for one DC, meaning I can work 0.5 fte. I need that for my own sanity, and my DC need it too. For context, AA plus DLA plus tax credits equals double what I bring home working 0.5 at the top of NHS band 7, so not a small amount. We are also fortunate to be able to access expensive specialist private therapy, via the adoption support fund.
Now I know that in some ways this is all like saying 'yep you're on the Titanic but at least you've got a really good suite', but you seem determined to think you are the most hard done by parent of a child with additional needs which aren't being met, when you've got more options than most. Also, unlike birth parents of DC with SEN, you've done prep and home study where you will have been told the likelihood of your DC needing extra support is high, and asked to demonstrate how you would deal with that, both emotionally and practically. Did you think it wouldn't happen to you, or just assume the system would have to sort it? Because forewarned is forearmed, and you would have been forewarned.
I get the frustration that there is no school which can meet your DS's needs, I really do. I get you don't want to HE, and shouldn't have to (I couldn't do it, a year of adoption leave had a massive impact on my mental health and if I tried to HE either of my DC at least one of us would end up dead or insane). I get the unfairness that other people get to go through life with kids who can cope with normal stuff and don't know what its like to have to battle for non existent suitable help for your DC. But it might help you and your DC to see and use all the extra resources you can access, which most parents of DC with additional needs can't.