In my experience there it's nothing to be achieved by delaying starting school for a child that is already showing delays and difficulties.
My child is August born (and autistic, though I didn't know this when they started school nursery).
They started school nursery in September 2020 and were literally 3 years and one week old. Part time was only on offer for the nursery so they did 3 hours a day for a full year.
They then started reception in September 2022, again just 4 years and 1 week old. They started full time hours straight away.
They have just started year one, age 5 years and 1 week.
Back in 2020 they were completely non verbal, couldn't hold a pencil, couldn't recognise any letters or numbers, socialising and playing with children was impossible, absolutely everything was a real struggle.
We made the decision to start nursery because we felt there was much more to be achieved there than being at home watching the gap widen. We agreed to see what happened and if they couldn't cope we would rethink.
Over the two years they spent at school in EYFS they have learned phonics and can now blend sounds to read basic words, they recognise all letters and numbers. They can add and subtract. They can write. They can answer comprehension questions from a book they have read.
Most important of all, my child can talk and can now have full conversations where we understand each other. All of this is because they were in an environment where they could learn from professionals and from other children.
They understand the school routine and what is expected of them, they can also play with other children with far less distress than before.
Although there are still significant delays and difficulties, they are equipped with everything they need to start ks1. It doesn't bear thinking about how they would be doing now had we not made the decision to send them to school and just see how they coped.