Does pre-chilling a cooler really make a difference in how long ice lasts?

Yes, pre-chilling buys you hours to a full day. Ice melts slower when it doesn’t have to cool down a warm interior first. With a good rotomolded cooler, you can gain 12-18 hours; with a cheap soft-sided one, maybe 4-6. It’s the cheapest performance boost — just use a bag of ice or frozen water bottles for 12-24 hours beforehand.