Yeah, I am no expert, Bertie if you want to read "The Whole Brain Child" and make your own mind up, that might be the one other book I would recommend.
I will say that I noticed the issues my DS was having at age 4/5 in part because that's when I and my sister developed the same issues. My exdh, similar at same age. I myself have been in therapy, and studied child development over the last few years as part of a professional qualification, so I followed the latest bits and pieces to cobble together something that works for my DS. It won't work for everyone.
The previous poster disagrees with me and I can see embedded her in words the idea that "if you just leave them to it, they will get it right". That was once the received wisdom, I totally get that, my own mum followed that approach, very much gentle parenting stuff. It works for many children I'm sure. There was a developmental psychologist called Piaget whose work was seminal and influenced a lot of pop parenting approaches, however if you look under the hood it's a bit scary how little of it makes any sense... he was very much "just leave them to it!" but research has shown that's not actually good for all kids.
Personally I am more on the Vygotsky side, with the zone of promixal development being the key concept that helps children learn at a pace that keeps them challenged and builds their confidence. You need an adult who loves the child and understands what they need to learn and will be their coach and mentor, and that is what gets them through and helps them learn how to learn. They break down the skill into tiny chunks and calmly and lovingly help the child build on each little chunk, gaining confidence as they go.
@Nettleskeins I can understand how my posts might horrify you - it does sound intense. In practice, it's not. The skill that was agony was handwriting - the rest have been sometimes frustrating but 90% rewarding, high-five moments. But they took work and there were tears along the way.
For example, the handwriting - I had to take control of that because I found out that his teacher had not required him to write anything for a year, and his fine motor had regressed about 2 years back to where it had been when he was 4 ish. He struggled to hold a pencil and had lost confidence at school ("I can't do the work anymore").
It was scary and intervention was necessary. Took 2 months of daily practice - and yes he hated it - and he is now back at grade level - and now we no longer practice handwriting daily. I tackled that skill only, for that 2 month period because I know that without it, he would lose more and more confidence at school. We went from "I hate school" to "school is so easy now!" after that 2 month period.
I do the same with other skills. I only tackle things that are easiest to learn at very early ages - e.g. swimming. Again, he learned ONLY to swim for a year - no other sports. And then ONLY to ski for three months. (We live in a country where skiing and skating are foundational skills for enjoyment of life - we are not in UK - without those skills you don't really go outside for 70% of the year).
His time is completely unstructured outside of the one skill we learn at a time. He isn't hot housed, despite how it may sound.
I'll take your point on music theory. He doesn't like doing it because his ear is excellent and he has got along without reading for a few years now (dad is a music teacher so he is exposed to it all)... however... if I try to tell him to stop working at it, he tells me that he will never give up, even if I do (eeeek). He will sit at the piano and cry with frustration until I come back to assist him / scaffold the next thing. When he gets it right, he absolutely BEAMS with joy and his day is made. That's all him - I just try to support him.