It's hard to know what's expected at each grade level, and where diminishing returns are in terms of learning, as well.
How, then, should we evaluate extracurricular resources?
For example: after school programs (like, Russian School of Math). They're expensive, and often require an annual commitment. But is there evidence they produce "successful" outcomes (how is that even measured). Are they snake-oil? Etc.
If you want to try something that is supposed to be more enjoyable without a time or financial commitment, check out Beast Academy. You can get the books cheaply, used, on Thrift Books or other such used book sites. The child is supposed to read a comic, by themselves if they are able, about a humorous situation in which a math skill wins the day. And then they do a small worksheet about that math skill. It’s low pressure and low anxiety inducing.