She went from a struggling, disinterested reader to a fluent one in a matter of weeks, increasing her difficulty level on her own.
That's just about how my DD learns too.
I really don't understand how it works but some children seem to have an internal compass when it comes to learning and if you interfere, they might withdraw and refuse to do anything for a long time. I don't want DD to believe that she can dictate her own learning across the board so it's been a fine balancing act. Outsourcing has made a difference for us. You don't want this to turn into Mom vs Child power struggle. I'd keep it fun and game-based as HM has suggested and provide her with many opportunities to choose her own materials but maybe you can also sneak in a few books that you think are at her practice level.
DD's piano teacher has 40+ years of teaching experience and she also teaches piano at a GT school so she's used to children who learn at a lightening speed. DD already has had 8+ months of private music lessons but she started DD at the very basic level, filling whatever gaps quickly. DD's done with Book A and moving onto Book B but the teacher decided today that DD can do the next level at the same time so when they are done covering the basics, DD would be ready to take off. Her teachers' words are "There is no point in holding her back or making her go through similar materials for repetition. We'll move at her speed." If only all elementary school teachers shared this point of view.
I never thought about compacting curriculum and filling the gaps simultaneously but DD was even more lively today than she has been during the last few lessons. DD has math gaps all over the place and for now, that's fine but when it comes to start filling those gaps, I'm going to see if I can use this approach.