The Bible says everyone goes to heaven at the same time.

I don’t think that’s clear in the text, and that’s historically been a major point of theological contention. I think the debate in the US 1800s over “soul sleep” and the affiliated quasi-cults that sprung up after the Millerite movement (Seventh Day Adventists, Jehovahs Witnesses) had very strong opinions on whether you go to heaven immediately, stay “asleep” in the ground until God starts resurrecting people, wait in some kind of cosmic waiting room for the resurrections, or if you are just flat out dead until God wakes you up. (Of course, JW’s are so committed to bad exegesis that they’ll die rather than receive a blood infusion, because “don’t eat animal blood” in the ritual purity laws of course means “don’t receive human blood infusions.”)

Think about Mormon baptism for the dead. Those people aren’t in heaven or hell (because at least the lower kingdoms of heaven aren’t even set up yet - all of us non-Mormons are going to be hanging out on Earth 2.0 when we die). Mormons are experts at genealogy because they’re trying to make sure that every great-great-great-great-grandparent they have gets a chance at salvation.