The best thinking that we have to date seems to say that if we do not have evil to compare to good then good cannot be defined.
No. It would be more appropriate to say that saying 'god is good' or 'good flows from god' actually says nothing about the nature of god (or good)
Digging this out from before (because every day is a learning day)
Theistic morality is subjectivist. If things are ‘good’ because god says that they are good then morals are arbitrary. Indeed, they are more arbitrary from a subjectivist perspective than our definition of morality because god (if it is in any way god like) is entirely unbounded by anything that would otherwise constrain us, or alter our path when making decisions.
So it robs ‘good’ from any definition. ‘Good’ is simply what something powerful mandates. If god mandates it, then ‘good’ means nothing. Saying ‘god is good’ is simply saying ‘god is god’. It says nothing meaningful about its actions because god would be ‘good’ no matter what it does. So that definition robs not only ‘good of its goodness’, but ‘god of its glory’. Why should there be praise for god if it would be equally praised even if it did the complete opposite? If what is arbitrary replaces what is just or reasonable, then all justice is, if anything, is what is pleasing to god.
So if things have to be ‘good’, then they must be good for another reason, if goodness needs to have value, then it can no more come from god that it can from us.
Saying that morality is actually grounded in god’s nature and expressed in its commands doesn’t avoid this problem. Whatever it was god’s nature to prefer would still be right by definition and still diminish the significance of moral terms. So saying god is good would just be saying that god also accords to its own nature which isn’t really an accomplishment. If it’s nature were different it would still be good. The wider issue is that theistic ethics are essentially ethically subjective; moral statements being made true by the attitude of certain people.
Which I think is what you might have wanted to say.