Hugs to you OP, we have all been there.
Our DS was diagnosed when he was 3.5 years (nursery flagged and we sought a private diagnosis).
His report described his needs as "moderate". The most notable thing was that his speech was delayed. He was also a late walker (which we now know is related to having low muscle tone - very common with autism).
He is now almost 5 years old and in reception in a main stream school. We cannot believe how much his speech as come on, we can have full on conversations (although they tend to focus around his special interests) - this was honestly unthinkable at 3.5years.
He is thriving at school, although he doesn't have a set group of friends yet (I don't think many of them do at this age). He loves the structure and loves learning.
It's not all plain sailing, his emotional regulation is poor. We've had play dates and birthday parties where he just melts down - but we are learning to understand his sensory preferences and coming up with coping strategies etc. We are really working on him understanding feelings, both his own, and others.
It's not been very long for us (only 18 months since diagnosis) but that "spikey" development that is common in autism can surprise you.
It is different for everyone. Let yourself grieve, DH and I were very down with the news initially - it lasted for about 6 months. I used my work's Employee Assistance Scheme for 6 sessions of counselling. We cried. My mother cried when we told her. But now we are in a place of understanding (and improved knowledge) and feel much more positive about our son's future.
In this process we have also realised that we are both most likely on the spectrum too - but we don't have the money to go private or the patience to go on the NHS for a diagnosis.