Even if the best things I've heard about Satan are true, that would not mean Satan is good - I'm not counting the claim that Satan is morally good while the description in non-moral terms shows otherwise or is at least silent, since I'm engaging in moral reasoning.
Moreover, the information about Yahweh is based on the Bible, and if that document is reliable about Yahweh, chances are some of the things about Satan are true as well, and those are mostly not good.
So, I would not be inclined to join Satan. If I were to take a stance against Yahweh and try to oppose him (with words, I guess), I would rather do it on my own, since the chances of success are zero regardless of whether I join Satan or not, and I prefer not to risk joining a potentially very bad guy.
Now, would I go up against Yahweh?
I have no weapons that might hurt him, so I could only denounce his evilness publicly - that is, continue denouncing it, as I already do even if he does not exist.
If I do that, I will spend eternity in Hell, punished by Yahweh.
But if I don't and shut up, I will spend eternity in Hell, since Yahweh knows my thoughts and knows I believe (I know, but apparently he does not know that) that he is a moral monster.
It is not the case that I have the freedom to accept Yahweh. I have the freedom to falsely claim that I accept Yahweh. But I cannot make myself believe that he's not a moral monster. So, it seems that I get punished either way.
Maybe I would try to make a deal with Yahweh: if he destroys me instead of torturing me forever, I will not expose his evilness. But I do not know whether such a monster would be amenable to making such a deal. He may well believe his own BS.
Tough choice.
It seems that regardless of what I do, I will be tortured forever by the evil monster Yahweh. Maybe the issue is to find the course of action that gives me the best (even if ridiculously low) chances. But it's difficult to make that assessment.