Op I could have written so much of this a year ago, although both of mine are ND but my eldest was super advanced developmentally and was so ready for school despite her challenges.
DS was diagnosed a little over a year ago at just under 3.5. He was non verbal until 3, and then when he did speak, was a gestalt language processor, and then with support/SALT input went on to use these to develop more spontaneous speech, albeit very narrative of what's around him (that car is red, the boy has a bear, etc) rather than anything expressive.
He has bowel/bladder issues and potty training seemed a distant dream that we couldn't even begin to imagine cracking, both with the ASD and the medical issues.
We applied to school last November and I genuinely wasn't sure if we could manage mainstream, but applied anyway and decided to give it a shot.
He is now nearly 4 and a half, and starts school in September.
The speech has come on in leaps and bounds. Expressive speech is slowly appearing, although we do sometimes default to scripts for that, but outside of expressing needs/feelings we're getting much more conversation and back and forth. He likes to process everything out loud, which sometimes helps us get an insight into his brain a bit sometimes, even if he can't do that in a conversation. He can follow 2/3 step instructions.
He potty trained himself for wees in November, and tried poos in December but we called it quits because it wasn't going well and he was getting really distressed. He tried again in February, at his own request, and cracked it there and then, not a single accident, although we're still working on coordination for wiping etc.
He still has his meltdowns, but we're getting better at knowing his triggers, and the warning signs, and have been able to communicate these to school. He is a bit more receptive to allowing us to help, and we can reason with him a bit more when needed.
And while some of his challenges are showing up more as he gets older (food/sensory issues) His personality shines through now. He doesn't always enjoy the typical things you'd expect, we were at a party the other day and he didn't join in a single party game, but he has some lovely friends. He is so kind and affectionate, loves to hug/kiss his friends goodbye. He's (mostly) very good at sharing, and loves to make people laugh. His problem solving skills are like none I've ever seen in a preschooler (probably from being non-verbal for so long). He loves his special interests, and can tell you all about them. He's nothing like so many of his peers, and our lives all look a bit different as a result, but he is an absolute joy, as I'm sure your DS is, and we're adapting. He's happy in his own skin, for now, and is so excited for school. I know it will bring new challenges, but I, like you, this time 12 months ago kept telling myself "yeah everyone says he'll change so much in a year, but surely he's not going to change enough to cope in mainstream" and he really has ticked off so much now, he just wanted to do it in his own timescale.