My daughter was not at grade 6 when she left primary school - she was certainly a good grade 4, learning violin too - at G5, and took up singing at age 12. As her piano teacher (and piano was her 3rd instrument in terms of choice) I found a range of repertoire at the level she was working at and gradually increased the difficulty, so that in fact her reading became much more fluent. This obviously benefitted violin and singing significantly too.
My daughter certainly needed encouragement to practise, but she didn't spend one hour on her piano per evening - that was not possible with school work too, plus other instruments, and some "down-time" for relaxation.
What does your daughter do in her hour's practice of the piano? Do you specifically supervise her? I would say, that she would benefit from playing a range of lower grade pieces - treating them as quick studies if you like, learning them by herself over a couple of weeks, so that her sight-reading was automatically being challenged regularly, but she wasn't having to do specific "sight-reading studies" which can be tedious, and rather off-putting for a youngster. In this way, her reading of music will become more fluent and she will be able to read through music more easily.
My daughter wants me to supervise but not micromanage... I usually give her indications as which scales / pieces etc to practise (we don't have time to practise the whole program everyday) + then tell her which specific parts to focus on... When pieces are new then I make sure everything is right (fingering, position etc) but lately she has even refused that I play the piece or section for her, she insists on discovering by herself.