With 23 of 23 samples, that’s a 78-100% 99% confidence interval (using an adjusted Wald method, which is good for small samples), so the sample size isn’t really an issue. The representativeness of the sample might be a concern, but for whatever the sample generalizes to, this is a high rate.

I think that kind of system is fine too.

While I agree that top tax brackets could use more granularity, it’s also important to note that it’s only income above that amount that gets taxed more. I’m personally okay with anything above that being taxed a lot and at the same (high) rate.