The first issue is that kids that age don't think that far into the future. Something that happens several hours away isn't motivating for them. It needs to be more immediate. Otherwise, it feels to them like they were forced to do things they didn't want to all day, then to top it all off on a completely unrelated note, they don't get dessert either.
Extrinsic motivation only works to a degree. It can jump start a child's motivation, but ultimately, she needs to want to do it for her own reasons. Since she already does okay "most of the time," I think it's unlikely to help much in the other times. You need to find out exactly why those other times are different.
Don't make the all-too-common mistake of assuming that needing to work hard as an adult means always needing to work hard as a child. Children learn to work hard by playing hard.
You have a lot of flexibility available to you as a homeschooler. Use it. If she's struggling with motivation, let her do something else for an hour or two and come back to it. It's funny how much more motivated they are when something is their own choice, even if you planted the idea in the first place.
For example, out of the blue a couple weeks ago my seven year-old started asking to learn about Ancient Egypt. He took to it with great enthusiasm. I assumed he had heard about it on a TV show or something. When I mentioned it to my wife, she said she had introduced the topic a few days earlier but he seemed utterly bored by it, so she dropped it. It only took a few days of mulling the idea around in his brain to go from utterly bored to actively seeking out information. By us not forcing the issue, he was able to get into a much more receptive state.
Especially something like playing an instrument requires a delicate balance of motivation and hard work. Missing one or two formal practices a week won't make much difference in her musical ability. However, I've seen many times how the cumulative effect of being forced to practice against your will hundreds of times over the next 12 years can result in never touching the instrument again as an adult.