My experience of 6 year olds is a bit out of date (my youngest is 11) but FWIW I think you do still need to do some of the school's reading scheme. Much of what we read is mechanical and for absorbing information, not just because it is a great read. Just as with everything, effort brings rewards. My own children were reading far, far ahead of their peers. We live in a remote area and their peer group is very small so they were working with those one or two years ahead. However, I encouraged them to do what they had to do at school and then we supplemented it with home readers. That way, the school had ticked their boxes and knew what level they had achieved but they also loved reading.
I have just finished reading "How Children Succeed" and it talks about "Grit" as an important part of character that will see children through to having successful lives. I think that teaching them to do what they have been asked to do and then go on and do more is a better life lesson than, "We will just read things that interest us."
I am a Secondary English teacher and we run three separate schemes. In one, pupils choose any book they want from home or the library and report on progress (allowing totally free choice). In the second, they choose a book in a group of 3 from a series of books (to allow them to work with others). The third strand is class texts which we discuss and analyse in greater depth. In that way they are learning both to read what they are given and to read what they choose.
To be honest, OP, I think you have made up your mind, however, and nothing we can say will change it.
If it was another subject (say PE) would you be saying to the school, "My child doesn't like badminton, she will just be playing tennis instead?" Or "Sorry, we don't really rate the Tudors in this household, we will be studying the Victorians."
If they are good readers, they will gallop through the books quickly enough.