First of all, I'd like to say I'm sorry that you're having to go through this journey. Even the people that get good results for their kids will tell you it fucking sucks. Not that it's hopeless or anything, but that it is SO hard and stressful. If you have the means, it really makes you appreciate it because the costs and hoops you go through add up. If you don't have the means to do everything, then it would make it even worse. We were also shocked by the number of kids that need help.
Our son was first flagged by us around 3 years old. The usual stuff. Meltdowns, speech delay, difficulty with transitions. We got him assessed and put in ABA therapy pretty quickly. Speech therapy, occupational therapy. We did diet changes like you've probably already tried, vitamin regimens. I feel like they all probably helped in their own way, but it's kind of hard to pinpoint what worked best. We really threw the kitchen sink at his issues. Fortunately, he progressed enough to go to kinder on his own and he's been in school since then. He did get put on a small dose of focalin in the first grade, but he's stayed on the same dose since then. He still has problems with emotional control. He cried at school much later than a typical kid would/should and that led to some bullying problems over time, but he's in 8th grade now and doing well. He's the school mascot and excellent student. I do wish for his sake that connecting with peers was easier, although most times he doesn't really seem to care. One thing that the first doc told us that has rang true is that his emotional maturity lags his age. She said at the time it would be 2/3-3/4 of his age. Has seemed about right. Anyway, I'm venting.
I owe all his gains to my wife. She did a ton of the legwork since I have a demanding job. You're going to need a strong partner. This stuff will strain you both to the max.
You haven't posted on this thread in a while, so I'm hoping things are going well.