OP, trauma can present in a very similar way to ASD/other LDs and you have already said that you suspect developmental trauma. The treatment and prognosis for each is completely different. A child can completely recover from developmental trauma with the right help (this doesn't apply to autism and some other LDs) - and incidentally yes, parenting and attachment are relevant, there are methods and therapeutic work which can drastically and quickly help recovery from developmental trauma. If you have looked at beacon house you will have seen their video about recovery from developmental trauma? If not I can link.
In relation to "parenting" it isn't so much you are a bad parent, it is more that what you are doing with this particular child may need to change.
I think you are right to go to go the GP route now, and you are also possibly thinking about the wrong thing here - but either way a solution will be expensive and /or require a lot of your time. And so go fund me might well help if you do get the money.
In summary, the treatment for ASD or other disorders and developmental trauma is completely different, with the right treatment for dev trauma you are looking at recovery. IME successful recovery involves a lot of your time input, you doing 1:1 therapeutic work such as long walks, sensory work but there could be quick responses with the right therapy and normality for your child and family, whereas for ASD you may be looking at special schools and once you go down that line you are looking at a life time of your child needing external supports.
My dc had developmental trauma and the recovery and difference over a period of a couple years were amazing once I started doing the rights things. At 3 the prognosis was that dc had severe LD and autism and would need a special school for the rest of his childhood, but this was incorrect - with a lot of therapeutic work within 2 years he was in a normal school and things improved year on year and he is now doing well in every respect, academically ahead in some areas.
Would an assessment for dev trauma/other be 2000 with beacon house?
If you go to "go fund me" with an actual diagnosis from beacon house and an idea of how much their therapeutic input would be, then you may be more successful?
I think you are right to go to your GP, these are just more things to think about.
If attachment and trauma are an issue then I hate to say it but weekly respite may well be contributing to the problems - in my experience.