Hi Ben. I'm an atheist and I also teach introductory ethics. There are many reasons to reject a belief in God. Yes, there is no visible/physical evidence for God, and yes, morality is also abstract. But we can have evidence for abstract things. Mathematics is abstract, but when we apply it to the world we observe, it works! No one can see 'infinity' or 'zero' but mathematics is still a useful system worthy of our belief. Likewise, abstract concepts are used in many ways, and when we have success using them, that brings us some evidence that they are true.

One of your concerns seems to be that without God, morality collapses into relativism. While that might be the case, it is not necessarily the case. For example, there could be rules that are perceived and understood by all humans because our brains work a certain way - a universal morality could emerge from that. A universal morality could be based on principles of helping one another and minimizing needless harm. We can act to foster love and equality. One does not need to believe in God to believe in these principles.

If your concern is that different people might, in practice, have different moral principles, well, that's true whether or not religion is in the mix. Jesus developed the Golden Rule and Confucious developed the Silver Rule, and they overlap a bit but are not the same.