DD9.5 will be starting 6th grade at a new school. The good news is that the new school's math curriculum is about half a year advanced compared to the public school system she's leaving. The bad news is that, while she didn't score high enough on the end-of-6th assessment she took to take 7th grade math, she already knows half of the material that will be taught in 6th grade math and, according to her teacher, will begin learning new stuff around December.

On one hand, you could look at the first few months of school serving as an adjustment period, since it will be a new curriculum with a new style of homework (she's going from Everyday Math to Saxon). On the other hand, she loves math and likes being challenged in math, and the first few month will be a time of no progress or challenge.

So, I'm looking into the possibility of finding an at-home curriculum that she can do afterschool at her own pace. I know there are several options out there, like EPGY, but what would be the best choice for intro to algebra for a kid who learns quickly and doesn't need a lot of reputation and would probably only have a chance to work on the program a few days a week? Does anyone have any recommendations?

Since it sounds as though challenge, more than linear progress, is what you're looking for, I'd suggest Alcumus. (I know, I usually do suggest Alcumus - but really, it's very good.) It is basically a lot of problems, often quite brain-teaser-y, with a topic tracker to get the level right for the student and some videos etc. as backup; if you wanted systematic teaching materials you'd probably want to buy a copy of their Intro to Algebra book as well, but if you're able to be around and help on demand you can do without that (we have the book, but hardly ever consult it; generally DS does a few problems on the bus).