You're right that prior probability gets very, very messy. It's a bit too abstract to actually be helpful to us.
So, then, all we can do is look at the evidence we do have. You're saying that the argument is one-sided; there is no evidence in favor of theism, at least no good evidence. I agree that there is a lot of bad evidence, and I'm still looking for good evidence. You've said you don't know of any. Thank you. That's what I wanted to know. In general I don't think it's healthy to believe the opposing viewpoint literally has no case.
You're right that prior probability gets very, very messy. It's a bit too abstract to actually be helpful to us.
You cannot escape the necessity of dealing with priors, however messy they are.
So, then, all we can do is look at the evidence we do have.
The available evidence supports an infinite number of hypotheses. How do you decide which ones to consider? That is your prior, and however messy it may be, you have to live with it.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.