I completely agree with this post. We weren't in a position 9 years ago to even consider private for primary, so both of ours went to state. Our ND eldest did struggle even with support, she's bright but easily overwhelmed. Our NT youngest is genuinely exceptional in maths, and his primary would brag that he'd complete all the work for the week during Monday's lesson, so he'd help them teach the other kids the rest of the week. Stretch goals aren't really widely used in a class of 30 as they just need to make sure as many kids as possible hit a certain level (I've heard this refered to as level 4 but not sure what that means) and those who are never going to achieve it, or can comfortably achieve it get lost somewhat.
We moved our eldest to an academically selective private for secondary and at the end of her second term she told us she couldn't remember when she last felt overwhelm, compare this to 8 months earlier when every day she used ear defenders, had to step out of class when it got too much, came home and fell apart in tears from masking etc. She is able to function in class 99% of the time as any NT child would. For us this is worth any amount of money (and we've made huge sacrifices to do private school, between us we now work 3.5 FTE jobs which is just the start). She's also emotionally supported now, she had some lovely teachers in primary but again just spread too thin to give her that time.
We ended up moving our youngest in year 5, if you'd asked me at that point to describe him I'd have said he was bright, but chronically shy (literally wouldn't talk to most people unless he knew them well) and his teacher was ex-military and shouted a lot so he used to just come home so scared. I'd call upstairs to say we were leaving in 20 mins and he'd burst into tears because I'd raised my voice above a very mild decibel. Having seen our eldest go through year 6 which was just an exercise in slogging for SATS to make sure the schools numbers looked good we moved as soon as we could afford the second set of fees so as not to waste that year of schooling and I'm glad because he's settled in and made friends before secondary, he has started to hear that he's good at things which is building up his confidence, and we've discovered he's not just good at maths, but actually working at a level around 5 years above where they'd expect. He will now offer opinions on things and talk to a stranger eg to order a meal in a restaurant, something he's never been confident enough to try until lately.
I will describe it as state schools are almost always doing the best they can with what they have, but what they have isn't enough. Not enough time, teachers, resources, support. A handful of teachers aren't good, but they're keeping them because they're so short staffed (a friend of mine works in a secondary near us and says they've not had a physics teacher in over a year now so have had to withdraw the a-level option, likewise my daughters year 8 friends in our local state haven't had a qualified maths teacher yet this year). Our sons class in primary had a class cello, everyone gets to bring it home one week a year. He'd never played other instruments not even the recorder class I remember from primary. He'd never been on a school trip except one outing to the post office in nursery. Now he does music weekly, art with materials other than paint and recyclables, DT where he actually makes a project out of wood, metal, electronics or textiles not just cardboard boxes, he does sport daily, he has access to a theatre and sports pitches not just a boggy field, he has after school options of chess, robotics, debating, fencing, rugby, windsurfing, horse riding, coding etc - there are a minimum of 8 clubs to choose from for year 3-6 each day, his primary had 4 clubs a week and none run by external providers with specalisms in that skill like they used to have when our eldest started in 2015. Every half term they have a class trip to support their learning, and 3x a year they have an optional overnight trip such as a sports tour, cultural visit or oudoor education within the UK (they add in overseas in secondary). It's just another world. We haven't encountered a single teacher at his prep who hasn't been enhusiastic about their role in developing the child, they don't seem worn down or snappy with him. His prep have recommended councelling though due to his extreme nervousness and tears if even minorly told off (things like 'you've left your trainers there Bob, might want to put them in your bag' has him shaking like a leaf) - they've described it as like seeing someone with PTSD.
You read a lot about how a bright child will do well in exams anywhere, and that may well be true, but they may not have a rounded skillset in presenting, debating, confidence that backs these up. It's that unknown unknown, how can you know what you're missing if you've never known it's there to miss? We toured the state schools first and one of them actually really impressed us, and then we toured the privates and it really is a different world. My husband works in tech and after talking to the state schools he said they all had the same issue, lack of time for the kids to get a strong grounding (one lesson a week or fortnight until GCSE, which fits with the state school we were at where they had one lesson actually using a computer per half term - this has been a big adjustment for my eldest to catch up on moving to private where she has a chromebook for every lesson), but also a curriculum that wasn't updated anywhere near enough to match the skills the tech sector needs now or is likely to need in 5 years, rather than what they did need 10 years ago. He said anyone of the kids at 16 from the private we ended up using could get a £45k+ starting salary with the bredth of what they are taught in a tech field.
I would also caution against reading too much into ofsted reports, the primary both mine went to has a big banner outside proudly proclaiming they're outstanding. Their last inspection was before our eldest was born and she's now year 8. In that time they've changed headteacher twice, and less than 15% of the staff are the same. They've also gone from a single form entry to a 2 form entry, doubling the school size. The 'outstanding' state secondary (recently inspected) near us has had 2 kids hospitalised in the last year due to violence. The benchmark for what makes a state school excellent is on a different scale entirely to what makes a good private excellent.
I intend absolutely no shade to anyone teaching in a state school, one of my parents is a state school teacher, but even they said to use private as soon as we could. I fully believe that state schools are doing the best job they can for as many kids as they can, but it's not enough and they should have more. Without being able to change the system for all we have opted to do the best we can for our kids and for us there isn't any question that smaller classes, a nurturing mindset, individualised learning and a huge bredth of oppertunities offered in the private sector is it.