My thoughts on the English still stand.
At KS3 English students cover a range of fictions texts, plays and novels; Shakespeare (in high performing schools I've worked in unabridged and withour modern translation); a range of non-fiction from 19th century to the modern day, range of writing in different genres and styles. A good KS3 should be about breadth and depth of study across language and literature (though I am aware some schools seem to do a full KS3 based on teaching GCSE skills questions in isolation, which sounds awful)
There are amazing texts we do in Y9 that you couldn't do in Y7 because of social and emotional maturity (even with very able children).
Equally, our top end KS3 (very bright!) struggle with unaren fiction and poetry analysis dropped down from Y11, not because they aren't bright enough, but because they haven't got the life experience and emotional maturity to pick up on some themes and ideas.
It is absolutely possible to stretch able students in KS3 English (I say that having taught classes of y8/9 who probably could pass the GCSE - but not get the 8s/9s, Y9 classes where I've given students literary criticism from a level and university texts).
I've taught some very bright children and anecdotally, the ones with parents who say they are too able to gain anything tend to be the ones who have a fixed mindset of 'i am clever' and really dislike it when given difficult work that they can't whizz through. It's almost like they love the image of being smart, love being given a task that is obviously harder than the other children, but they are obsessed with the idea smart people finish first, lack resilience, and they get annoyed when they aren't first done or they see others potentially threatening their 'top place'. The mindset can be very much 'i am smart and found that work difficult so the teacher can't teach otherwise I'd have found it easy'.
By all means, opt to home educate if it's better all round but you may need to loose the arrogance of 'school is just for socialising because my child is so bright they won't learn anything'.