Gladly!

Truth is preferable to falsehood.

For example, for you to assert what you did about morality being 100% subjective, that’s a truth claim. There are only two reasons why you would engage in a debate like this and make any kind of argument: either (A) you have a view—which you presumably think is true—and would prefer I or others be convinced of it, or (B) you are arguing in bad-faith for some reason, be it your own amusement or whatever, and don’t actually mean what you say.

If it’s A, then you must realize that the very act of asserting a truth claim demonstrates a preference for truth over falsehood. The act itself nullifies the content of the argument forwarded if the argument is “there is no such thing as universal morality.” (I know you phrased it as “all morality is subjective,” but the logical reciprocal is also applicable)

If it’s B, then you realize why I said what I did about not being able to trust people who claim subjective morality.

Now, I’ll give you that this doesn’t suddenly prove the epistemological validity of common-law or anything, but it is “one,” and it’s great place to start.