For many years, this was more or less how we home schooled. Finish all your work for the week by Thursday afternoon, and Friday is yours to use as you wish (short of all day on a digital device). I plan on roughly three hours worth of work per day, with the rest for choice activities. If the work isn't finished because it was too difficult, then I back off on expectations. If the work is finished in much less than three hours, then I bump up the pace. (If the work isn't finished because you spent all day on Scratch, then I guess you just used your choice time for the week!)
It turns out that most of our children are not extrinsically motivated, so "rewards" are less effective than personal relevance of the actual topics/tasks.
When the one who attended school was in school, the promise of free time was not nearly as motivating as the socializing that occurred during work time, which prevented early completion of work. That was also when there was a huge mismatch between the level of classwork and the child's challenge level, though.
