You sound exhausted, and no wonder.
I agree re the ECHP, it should include all the therapies your child needs including specialist school provision where needed. I know how it is when you can fund this for yourself, it feels easier and quicker to just sort it out. The problem is though that the supports aren’t joined up in the way they would be if coordinated through an ECHP. I’d also echo thinking about senior school placement - the right specialist placement will be life changing for your child, and will help with some of the issues around having support eg other parents whose children have complex needs who will get where you’re coming from in a way your pre-child friends probably won’t.
Is the private SALT making a difference? They should be able to clearly explain what they’re working on and expected progress, what are your hopes for the therapy?
Do you have any understanding of why his development is so limited? What assessments have been done to establish what’s going on with him? The reason I ask is because many of my DDs difficulties were put down to trauma (and there’s lots of trauma to deal with) but there were also physical issues that caused problems with toileting, speech, language development and learning. When those were finally dealt with (after much argument with health professionals), things got much easier. While she’ll always have difficulties things like her being able to toilet independently make a massive difference.
I think too adjusting your sense of his capacity will help, he sounds developmentally delayed, so things are going to take longer and look different for him. It sounds like you weren’t aware that he’d have such challenges and life isn’t how you thought it would be - which is very hard indeed. Celebrate the things he is able to do, take pleasure where you can and make sure you both get time for yourselves, from a coffee in the garden to a weekend away with friends so you can glimpse the person you are without the challenges you’re coping with now.
Id also access counselling, I sliding usually suggest EAP counselling though - that’s helpful for short term, solution focussed support, you need something more relational someone who will hear how hard it is, and help you mourn for your old life and the child you hoped for.