Here are the course sequences:
https://basisschools.org/student-handbook-course-sequences
I don't know if they are planning to make any changes. Their focus does seem to be on STEM but do seem to at least have a decent emphasis on grammar, literature and composition as compared to a lot of the other schools. I was impressed that their students won the Library of Congress "Letters About Literature" contest, along with having several other Basis students as finalists.
Last year they did remediation, they also do a STARS program to give kids a boost. Most other DC charters I am aware of do not do anything like that.
Sadly there are a handful of parents who don't seem like they are on the ball - for example saying they don't know how their kid is doing or it being a surprise when they fail on precomps or comprehensives, since you can get a good idea of where your kid stands and how he or she is doing every week via the CJ. Basis is not a miracle school that will magically turn young Johnny into the next Einstein. It's a challenging and demanding school that does take some solid commitment and support at home from mom and dad - some families aren't paying attention and are learning that the hard way.
I also don't think the numbers held back will accumulate as it would have to take a really special kind of obstinate and obtuse to allow your child to stay behind 2 years. Especially so, when Basis give second chances by allowing students who failed comps to work on a packet over summer and retry.
They are demanding and challenging but one certainly cannot accuse them of not helping, not trying, or just leaving kids to flounder with all of the supports that they give - far more than most others give. But, there's only so much that one can expect them to do given the limited resources they have. There's much that the student and families need to be bringing to the table too, in terms of academic support.