It makes perfect sense, my DD sounds similar in that age can learn numbers, times etc by rote but couldn’t tell you what they mean - which means concepts like addition, multiplication etc are almost impossible for her to grasp because she can’t learn through the usual strategies.
Given the complexity of his needs Im surprised school are saying he’s ok. Does that mean his behaviour is ok, because his learning must be impacted given what you’re describing.
Parenting courses honestly aren’t going to do much here, he needs educational support and therapeutic work - the first stop is always that children just need secure, attuned parenting, which flies in the face of all the research around developmental trauma and then makes parents feel like shit when they’re doing their best and their child still struggles. And the child then hits crisis in their teen years and it’s much harder to help them at that point. It’s not fair to parents or children.
I wonder if having a plan of attack, so to speak, might help. Break things down into immediate, medium term and longer term - you sound exhausted and overwhelmed which is no surprise really.
In your shoes I’d look firstly at what financial help you can get to allow you to reduce your hours - check “entitled to” with various options in terms of income eg your job alone, your DHs income alone, variations of part time work. That might give you a sense of what you can afford to drop. I know how hard it can be to start looking at benefits but your DH needs to understand that something has to give, and it’s either going to be your health or your child’s future neither of which are expendable.
I’d also start really challenging the school about what they mean when they say he’s ok, and how they plan to teach more abstract concepts etc. Ask them specifically what they’ve done with the OT recommendations and explain you’re starting the ECHP process regardless of whether they support.
Id then contact post adoption support and ask specifically for therapeutic support and see what they come up with.
In the meantime keep things at home as low key as possible, pick your battles and get as much down time as you possibly can. You’ve a road ahead of you and you need to be well enough to cope.You’ve done amazingly well to get this far.