Supporting your point with another dictionary is a good way to go beyond statement of opinion!

But the Cambridge definition includes "harmful or unfair things that people say, do, or think based on the belief that their own race makes them [better than other races]". It even includes as examples being targeted by "overt" or "casual" racism--how would being called a racial slur, for example, not count?

So this one doesn't support your perspective either.

This doesn't mean that your perspective is necessarily wrong, just that this dictionary also didn't help you support it. (Both dictionaries could be wrong.) But it also doesn't establish that it is correct: this is a matter of actual usage and/or some clearer argument.

(You make a good point that there is a range of degrees of severity, though, and it minimizes the severity if you focus on the "weak tea" parts of it. But there's a big housecleaning needed to remove the "weak tea" stuff--all the "microaggressions" are pretty much that by definition.)