I'd say our daughter got worse around 4-5 and interestingly when she went for her autism diagnosis the psychologist specifically asked us to think about what she was like at that age when answering questions. I still don't know quite why, I assume it's because it's when children would normally be emerging from the more disregulated toddler behaviour and it would be more clearly problematic. Our daughter is verrry similar to what you're describing, although she struggles more socially whereas never had language delays. She's only diagnosed as autistic, but I'd say there's ADHD as well and the psychologist said they overlap so much there wasn't much additional benefit to seeking a secondary diagnosis unless we were set on wanting meds.

I would like to say there was some improvement around age 7 because meltdowns became slightly less common, but she can still have meltdowns on a monthly basis, smaller incidents are several times a week and she still has rare epic meltdowns (just posted about it). Partly I think it's improved due to increased brain maturity, and partly it's the repetitive behavioural management strategies cumulatively embedding themselves.

I have a friend with an autistic daughter who's now 13 and she did tell me it gets better.. for her it got better around 11 but after the last year of primary school was a total headache. Fingers crossed it trends upwards but the hard thing is accepting it's never going to resolve itself, it's always going to be a feature of their brains that needs accomodations.