> Fortunately, there is no truth of the matter, so anything goes as long as we're internally consistent.
I don't think that what you said is internally consistent. Now, we may disagree about exactly what you mean, but it sounds like you are putting forth some form of moral relativism, which has appeared in various forms over the millennia. Many ways of phrasing moral relativism have logical inconsistencies in them.
I think the "anything goes as long as we're internally consistent" is a statement that any ethical system, E, is correct if and only if E is internally consistent.
And "there is no truth of the matter" is saying that "no ethical statement is true in all systems of ethics".
Now, is the "anything goes" statement an ethical statement? Is it true in all systems of ethics? If both, then this is a contradiction. If it is not an ethical statement, then what does "anything goes" even mean? If it is not true in all systems of ethics, then how can you defend it without begging the question? I'm not trying to enumerate all the possibilities, but just illustrate my skepticism... I don't see an easy way out here. It's not impossible, just really quite tricky, and if you value being internally consistent, maybe you should dig deeper.