I like the Joseph Campbell approach to mythology (or religion/spirituality if you prefer). Metaphors are real in the sense that they are actual mental artifacts which serve a valuable purpose. They really can help us relate to the world in constructive ways. But it’s important to recognize that they are metaphors. They are true in the poetic sense of the word rather than the scientific, historical, or even philosophical sense. The feelings these metaphors produce are real, but feelings are not evidence that supernatural beings exist or that miracles like raising people from the dead really took place.
The problem is religion doesn’t generally see itself as metaphor. Your solutions to the diversity of religion problem would be seen by most faiths as heresy. If in fact most religions/believers saw themselves as all climbing different sides of the same mountain in a quest to get at some deeper truth, few if any agnostics, atheists, humanists, or unaffiliated spiritual seekers of various types would really have any reason to view religion as problematic. But most religions don’t see it that way. They generally take their metaphors as literal truths, even if to somewhat varying degrees. The more liberal faiths that are far more tolerant of diversity of belief have low membership numbers for a reason: they don’t offer the same illusion of certainty orthodox religions do.
I admit, I really don’t see what belief in a god or gods adds to the quest for truth. If something is true, it will be true whether there is a god or not, so I prefer to just focus on the truth and skip the middledeity. If god has a reason for proclaiming something immoral, why not just go to the reason? If god doesn’t have a reason, then he/she/it is just being arbitrary. Is slavery immoral for a reason or is it immoral because a god(s) is feeling particularly humane today? I think we would all prefer the former to the capriciousness of the latter.
Regardless, I do believe our religious traditions do get at moral truths upon which we can all agree. But when a believer takes a literal stance, the only options are to either ignore them or address their literal claim. Literalism backs everyone, believer or not, into a corner. As an agnostic, I confess there’s no proving god either does or does not exist, but if someone insists the existence of their god is literally true, even if only in an alternative universe, they are making a factual rather than a faith based claim. Facts can, at least in principle, be tested. If you tell me your god literally exists somewhere, the logical followup question is ‘how do you know?’ If you tell me you have faith or prefer to live as though there is a god, that’s a much softer claim that implicitly recognizes there is reason to doubt it is literally the case.
Believers cannot treat their religion as fact and then get upset when their claims are subjected to philosophical or scientific challenges. Factual claims are the realm of science and, to only a somewhat lesser degree, philosophy. True faith and metaphor, on the other hand, admit to uncertainty and nuance in a way science and philosophy are not intended to. We don’t take facts on faith. Religion too often assumes it knows what’s unknowable, thus inviting criticism both from other faiths making contradictory literal/factual claims of their own and nonbelievers alike. It’s something of a paradox, but for religion to work as I think you and many others intend it will have to take itself far less seriously. Buddhism and Taoism have, I think, accomplished this to a fairly large degree (though both are arguably closer to organized philosophies than actual faiths), but the monotheistic traditions are all built on the premise they are the only true religions out there. You’re not going to heaven except through Christ, etc.