Does the 5W charger prolong battery life on the iPhone 15 Pro Max?

If your phone is heating up while charging, and that could be any charger, but likely a higher wattage charger, then it's probably not good for your phone. Now there is no charger in the world right now that can prolong battery life, although some can lessen the decline in that battery life. A 5W charger is quite good to use if you've got the time, because it will almost never cause overheating, so you're definitely pretty safe using it, but that's not to say that a 20W charger is terrible all of a sudden, because those don't necessarily damage battery life, the risk is just higher obviously, especially combined with a warm surrounding climate.