Even if there are, wouldn't the process of finding them effectively mirror moral relativism?..
Assuming that slavery was always immoral, we culturally discovered that fact at some point which appears the same as if it were a culturally relativistic value
It is a useful exercise to attempt to iterate some of those "discovery" processes to their logical conclusions, rather than repeatedly making "discoveries" of the same sort that all fundamentally rhyme with each other and have common underlying principles.
if morals are absolute then why exclude some of the commandments?
The alternative is that you get outpaced by a competitor which doesn't bother with addressing ethics at all.
If you are done solving that question, next prove that the book you favor is from god. There's a lot of competition for this claim as you know.