@IHateCoronavirus, you sound brilliant, wish you were teaching my kids!
Sounds like the OP has a kid who is well ahead of the pack.
Boredom can be a big factor but usually in the later primary years. In Reception it is just fun and play. In Y1 and Y2, it seems to me that a lot of the real learning happens 1:1 at home; reading, spelling, basic maths. In a class of 32 or more most of the time is spent on crowd control not quality teaching and so it is up to parents to ensure standards are reached. I viewed my children's school hours as an extension of Daycare until Year 3, and we did do the school reading and maths and writing practise at home, but also read our own books too. Wish now I'd done more maths.
In year 3 they do try to settle the kids down to work, but it is a bit painful. Whole class teaching differentiated at three levels, essentially mirroring working below, at or above standard. The standard is not hard for able children. No kids can learn at their own pace in maths, but free readers have a slightly better time (although zero support) and creative writing is well supported and taught in the curriculum. At my child's school they don't even have free access to the library, just a selection of allegedly age-appropriate books in class most of which my child has read years ago, or are so dumb she isnt interested (you know the kind I mean, jazzy covers and titles like, "The fart machine from outer space". We use our local library to expand reading horizons. Extension work in maths exists if your child's teacher is ace, or if you moan a lot (I havent). Able kids are used regularly to support the learning of the less able, which is okay, but moves everyone to be average. Boredom and apathy sets in, there is no expectation of high attainment. The kids who want to sit 11+ have to get tutors or parental help because no state school teaches ahead on the syllabus, so if you want to cover algebra or ratios properly before year 6, you're on your own.
I do think English is taught to a much higher standard these days, and the nature of creative writing is necessarily unlimited. So if the exercise is to write a persuasive letter to your MP about environmental issues, an able child can really get their teeth into it. The wider curriculum is also very broad and satisfying; your 9 year old might already be an expert on Ancient Greece and its myths, but it is unlikely they have created a poster comparing the pros and cons Spartan and Athenian society and political systems, and then participated in a group debate to decide which they'd prefer to live in.
But maths is depressingly unambitious in schools. My DB has bright but definitely not genius kids, whose lives revolve around TikTok and football like many kids. But as DB has Oxbridge degree in maths, he taught them the principles of arithmetic - a real understanding of number - by the time they went to school. By age ten, the eldest calculated how many seconds there are in a year whilst driving to the station with me, just for fun. His little brother liked to be quizzed on multiplication and long division age 6. Now age 15, the eldest mastered differentiation in a single sitting and does maths olympiad and is starting a special pre-university foundation course offered to maths high flyers. He is now considered to be gifted, but really he was just exceptionally well taught from an early age. The moral is, if you steadily home-coach your child in something you are very able to teach and they enjoy, they are more likely to go further. Not a very surprising conclusion.
So - use school for socialisation and breadth of syllabus. Read the curriculum and list of expected standards, so you know what is done in each year. Teach patience and good behaviour so boredom at school doesnt turn into misbehaviour. Then all the stretch comes from home. You could focus on academic stretch or divert into extracurricular such as drama, cubs/brownies, coding, music, or probably some of both.