I have a DS12 who looks a lot like this in school. He's a natural mathematician who flies through math many years above grade at home, but was barely passing math in primary school. For what it's worth to feed into your thinking, here's what we have figured out so far is going on.
What's caused bad grades: He seems to have a combo of inattentive ADHD and writing issues (both physical and cognitive); oodles of anxiety. In younger grades, math was a lot of writing - which he never completed, so never got to move on to more challenging work - and a lot of repetitive, basic computations - which set off the ADHD and anxiety something awful. We're guessing fine motor problems from hyper mobility (so writing hurts), and likely expressive language deficits. He also has a weakness in computation, which may be memory retrieval issue, not sure. This wasn't apparent until grade 6, when the work got hard enough and other kids fast enough that his teacher started to notice he was spending too much time in tests working out basic calculations instead of having them memorized, and was introducing a lot of errors as a result (errors also come from not being able to read his own writing). (He keyboards all work except math.)
What caused good grades: His math grades leaped up into As in grades 5 and 6, when for the first time he had teachers that allowed him to spend most of his time off curriculum. Not overly enriched, and certainly not accelerated, but at least interesting. Key success factors: more conceptual math, allowed to do other stuff while concepts he already knew were being taught/ practiced, no worksheets, and far less writing (show your work using math, not "explain your work in multiple sentences/ paragraphs" on every question.) The harder the math, the better DS does; he does far better at contest math and AoPS than school work.
When he was younger, I would have said he was a great candidate for acceleration (alas, not an option around here). As he got older, though, it became obvious that while he needs (but won't get) radical acceleration in math, his writing and executive function skills are so weak that full-grade acceleration would not have been a good idea. Grade 7, this year, is becoming a painful struggle with these issues. Moving to a gifted program in grade 6 brought all his other grades up - being far more engaged helped him work through his disabilities. But now the sheer output and executive function expectations in that program are overwhelming him, and grades are now plummeting. Like you, we have also found at-home math enrichment increasingly difficult in the last two years because of all the incomplete writing school work (never math!) coming home.
Our pathway may look nothing like yours, but I thought I'd share as food for thought in case something resonates. The huge challenge with these kids is that what they need looks so different from year to year. The impact of their strengths and weakness depends much less on the kid than on the environment they are in. Ideally, 2E kids are best placed according to their strengths, while supporting their weaknesses: not by holding them back to their weakest area. But that may be easier said than done if you're in a system that can't deal with a kid that uneven. So all that to say, if you are looking at acceleration, you may want to ask some very pointed questions about how they could ensure challenging material without overwhelming expectations for output or executive function or other 2E issues.
