My son was easily diagnosed, a preliminary diagnosis (with hearing, genetic testing, SALT & portage to see him) on the first visit in April. I finally read the report from then in October when he was given a firm diagnosis and I was so surprised at how absolutely obvious it was! I was still unsure and "suspected" and had been shocked that one paediatrician had said it so blatantly after one visit! But in hindsight, it really was obvious.
And as a diagnosis, I LOVE that it's there in black and white. He's starting nursery after Christmas and so they all know, they can apply for funding for a 1to1 for him, from their earliest interactions with him they know. I can specify so many things that won't be taken as "fussy parent" but as firm things to make his time enjoyable and to actually allow his teachers to enjoy him without having a confused and distraught child on their hands. And that will allow him to reach his huge potential.
I understand your worries though. My ds1 is less obviously on the spectrum, but it's obvious to me now he is. Everyone used to mention how hyperactive he was as a child. But they weren't restrictive behaviours. However, he STILL at 10 and nearly 5 feet tall throws himself around the living room, no matter what the sanctions he cannot help throwing himself around. So what's appears normal for a small child can actually be a subtle manifestation of behaviour. I wish I'd known things back then because while he's in a fabulous school now his last school was disastrous for him and if he'd had a "label" we all would've known why a supposedly typical child was getting into so much trouble, having meltdowns on the way home and screaming and wrecking his room and wanting to be dead, at 7. But I'm still reluctant to get him "labelled" even though he may have problems transitioning to secondary. It's completely irrational.
It can be really hard to see things in our own children that are obvious to others. It's only since ds2 was diagnosed I realised ds1 has never done imaginary play. He seemed to play, but since Portage explained to me about imaginary play, well, no. He used to line his cars up or crash them together to see who was the winner.
The school he's at says that children with autism do very well there so no wonder he's thrived, as do children without autism! In fact they get as good SATS results as the school which actively discourages SN children. One of those schools has fantastic inclusive teaching, the other relies upon the fact it has a reputation which filters out parents who aren't hugely involved and so relies upon outside factors for its "success".