Yeah, the concept is nice, but it tells me that the Big Bang doesn’t explain what happened before it (the leading hypothesis is that the Big Bang started time, so there is no “before”)

Which is entirely correct. Time as we know it is an "inside" parameter of our universe, and therefor any causality only exists inside our universe, too. Because causality always contains a temporal element as in "Event A happened, which caused Event B later". We cannot make any assumption of "before the big bang" and therefor no assumption of "what caused the big bang" either.

At least not in any way we could relate to.

the common myth about Daddy Longlegs being highly venomous

Quite a childrens tale, even back then. Two reasons for it: First, the "Daddy Longlegs" has no ability to bite us. Even extreme thin parts of the skin, e.g. the lips, are still way to thick for it to penetrate with its teeth. Second, even if it could inject its venom (which really exists!) it would need to inject about half a cup of it into a grown adult (IIRC about the amount, it could be a quarter cup or a whole cup or something, but still in the range of "thousands of total spider weights").