You absolutely have to collect medical evidence, including diagnosis at the earliest opportunity. There are backlogs and delays. Don't add to delay.
If DS2 had not been diagnosed his access to support (illegal as access is supposed to be need led) would have been delayed and he would not have had an EHCP whilst still at primary. As well as ASD and ADHD he had speech delay and was written off as biddable but stupid. Primary called in EP expecting low IQ but it turned out he was 99.8 level.
Without an EHCP, He would not have had priority access to secondary schools.
When secondary failed he, without an EHCP, would not have had access to internet school and tutors funded by the LA. He would not have achieved 11 GCSEs with good enough grades to progress.
Without an EHCP he would not have been entitled to support and extra time at 6th form college and achieve A*, A A in maths, further maths and physics.
Without his EHCP, medical history and previously supplied support he would not be entitled to DSA and extra time at uni.
Support at school is crap. But it gets better if you can navigate that. You need medical evidence to do that. Don't let schools convince you it is of no value.